upgrade from mongodb 2.6 to mongodb 3.0.12 getting authentication error in rails + heroku

upgraded mlab from 2.6 to 3.0.12. am getting following error in console while trying to connect to mongodb

Moped::Errors::AuthenticationFailure: The operation: #

please find my Gemfile.lock below

rails (3.1.1) ruby 1.9.3p551

moped (~> 1.2) moped (1.5.3)

mongo (1.11.1) mongoid (3.0.23) mongo mongoid