Installing bundler gives me "you don't have write permissions" error

I had used ruby version manager (rvm) to install a new version of ruby. Running the command rvm rubiesshows all the version of rubies that are installed. The list is given below.

rvm rubies

   ruby-1.9.3-p551 [ x86_64 ]
   ruby-2.1.5 [ x86_64 ]
   ruby-2.2.4 [ x86_64 ]
 * ruby-2.3.0 [ x86_64 ]
   ruby-2.3.1 [ x86_64 ]

# => - current
# =* - current && default
#  * - default

I tried to run the command bundle exec rake rb:mgirate and I get the following error.

/usr/local/rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/rubygems/dependency.rb:319:in `to_specs': Could not find 'bundler' (>= 0.a) among 5 total gem(s) (Gem::LoadError)
Checked in 'GEM_PATH=:/nix/.nix-profile/lib/ruby/gems', execute `gem env` for more information

The error states that bundler isn't installed. I ran the command gem install bundle and I get the following error.

ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /nix/.nix-profile/lib/ruby/gems directory.

It says that I don't have write permissions. I googled the error and it has something to do with updating ruby and ruby version. The reason why I updated to a new ruby version was because heroku run rake db:seed wasn't seeding data in production. I though it might be due to compatibility issues. Therefore, I decided to update to the new version of ruby. The servers, postgresql and everything else works fine. I can't run bundle because I can't install the gem. Any hep would be greatly appreciated. Thanks!