Updated source in Gemfile

:rubygems is deprecated as of Bundler 1.2.4
Updated README
Updated airbrake dependency to 3.1.8
Use only symbolic group names in Gemfile
Updated newrelic_rpm dependency to 3.5.6.55
Updated multi_json dependency to 1.6.1
Updated mongo dependency to 1.7.0
Updated mongoid dependency to 2.6.0
Updated dalli dependency to 2.6.2
Updated bson_ext dependency to 1.7.1
Updated jquery-rails dependency to 2.2.1
Updated sass-rails dependency to 3.2.6
Updated rspec-rails dependency to 2.12.2
Updated mocha dependency to 0.13.2
Improved error handling and reporting during imports