RubyGem version error: activesupport(2.1.0 not = 2.0.2) (Gem::LoadError)

need to change the hardcoded gem version in the environment and remove
old gem then load the new gem

1. check in config\environment.rb for RAILS_GEM_VERSION

  RAILS_GEM_VERSION = '2.1.0' unless defined? RAILS_GEM_VERSION

2. run gem cleanup -v
             gem update