Upgrade rails and json due to security vulnerabilities
Puts therubyracer in asset group and only install if on linux.
Updates therubyracer.
Updates mocha.
Merge pull request #742 from wilkie/bugfix/therubyracer-only-on-linux

Bugfix/therubyracer only on linux
Fixes mocha deprecation warning.
Refactors Rakefile for tests such that no environment is loaded.
Merge pull request #745 from wilkie/bugfix/faster-tests-through-rakefile

Bugfix/faster tests through rakefile
Ensure ARGV[0] is present when running tests
Merge pull request #746 from dtykocki/master

Fix for undefined method `start_with?' for nil:NilClass in Rakefile