Unable to migrate database in EC2 using capistrano : FATAL: role "ubuntu" does not exist

While deploying Rails 3 application on EC2 with RDS(Free instance) we are getting following error.

* executing "cd /home/ubuntu/integrity_matters/releases/20150318063048 && bundle exec rake RAILS_ENV=production  db:migrate"
    servers: ["52.10.245.227"]
    [52.10.245.227] executing command
 ** [out :: 52.10.245.227] /home/ubuntu/integrity_matters/shared/bundle/ruby/2.2.0/gems/activesupport-3.2.17/lib/active_support/values/time_zone.rb:270: warning: circular argument reference - now
 ** [out :: 52.10.245.227] 
 ** [out :: 52.10.245.227] rake aborted!
 ** [out :: 52.10.245.227] 
 ** [out :: 52.10.245.227] FATAL:  role "ubuntu" does not exist

deployment.rb

require "bundler/capistrano"
require "rvm/capistrano"

server "IP_ADDRESS", :web, :app, :db, primary: true

set :application, "integrity_matters"
set :user, "ubuntu"
ssh_options[:keys] = ["~/.ssh/im_ec2_free.pem"]
set :deploy_to, "/home/#{user}/#{application}"
set :deploy_via, :remote_cache
set :use_sudo, false

set :scm, "git"
set :repository, "git@github.com:BoTreeConsultingTeam/IntegrityMatters.git"
set :branch, "master"


default_run_options[:pty] = true
ssh_options[:forward_agent] = true

after "deploy", "deploy:cleanup" # keep only the last 5 releases
after "deploy", "deploy:migrate"

namespace :deploy do
  # Reference: http://ift.tt/1MIWfEE
  task :symlink_assets, :roles => [:app, :db, :web] do
    run "ln -nfs #{shared_path}/assets #{deploy_to}/assets"
  end
  after "deploy", "deploy:symlink_assets"

  # Reference: http://ift.tt/1FvEqZj
  task :cold do
    transaction do
      update
      setup_db  #replacing migrate in original
      start
    end
  end

  task :setup_db, :roles => :app do
    raise RuntimeError.new('db:setup aborted!') unless Capistrano::CLI.ui.ask("About to `rake db:setup`. Are you sure to wipe the entire database (anything other than 'yes' aborts):") == 'yes'
    run "cd #{current_path}; bundle exec rake db:setup RAILS_ENV=#{rails_env}"
  end

  %w[start stop restart].each do |command|
    desc "#{command} unicorn server"
    task command, roles: :app, except: {no_release: true} do
      run "/etc/init.d/unicorn_#{application} #{command}"
    end
  end

  task :setup_config, roles: :app do
    sudo "ln -nfs #{current_path}/config/nginx.conf /etc/nginx/sites-enabled/#{application}"
    sudo "ln -nfs #{current_path}/config/unicorn_init.sh /etc/init.d/unicorn_#{application}"
    run "mkdir -p #{shared_path}/config"
    put File.read("config/database.example.yml"), "#{shared_path}/config/database.yml"
    puts "Now edit the config files in #{shared_path}."
  end
  after "deploy:setup", "deploy:setup_config"

  task :symlink_config, roles: :app do
    run "ln -nfs #{shared_path}/config/database.yml #{release_path}/config/database.yml"
  end
  after "deploy:finalize_update", "deploy:symlink_config"
  after "deploy", "deploy:assets:precompile"
end

database.yml

production:
  adapter: postgresql
  encoding: unicode
  host: HOST_COPIED_FROM_RDS
  reconnect: false
  pool: 5
  user_name: imdeploy
  password: ****
  port: 5432

Please not that if I connect to EC2 server via ssh with ubuntu user, I am able to connect to RDS from EC2 server with following commands

$ psql -h HOST_COPIED_FROM_RDS --port=5432 --username=imdeploy --password --dbname=prod_rds_integrity_matters

Further here is list of DB and permission details

prod_rds_integrity_matters=> \l
                                          List of databases
            Name            |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges   
----------------------------+----------+----------+-------------+-------------+-----------------------
 postgres                   | imdeploy | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 prod_rds_integrity_matters | imdeploy | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 rdsadmin                   | rdsadmin | UTF8     | en_US.UTF-8 | en_US.UTF-8 | rdsadmin=CTc/rdsadmin
 template0                  | rdsadmin | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/rdsadmin          +
                            |          |          |             |             | rdsadmin=CTc/rdsadmin
 template1                  | imdeploy | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/imdeploy          +
                            |          |          |             |             | imdeploy=CTc/imdeploy
(5 rows)

prod_rds_integrity_matters=> \du
                                  List of roles
   Role name   |                   Attributes                   |    Member of    
---------------+------------------------------------------------+-----------------
 imdeploy      | Create role, Create DB                        +| {rds_superuser}
               | Password valid until infinity                  | 
 rds_superuser | Cannot login                                   | {}
 rdsadmin      | Superuser, Create role, Create DB, Replication+| {}
               | Password valid until infinity                  | 
 rdsrepladmin  | No inheritance, Cannot login, Replication      | {}

I also tried setting set :use_sudo, true in deploy.rb but still getting same issue.