Add ability to specify custom rake command
Refactor ability to specify custom rake command
Do not add "rake" if it is already in custom cmd
Check if default guard-migrate rake tasks are already specified in custom cmd
Update rspec-expectations and rspec-mocks gems
Fix Rubinius issue with autoloading while running tests

The issue is described in: https://github.com/rubinius/rubinius/issues/2934
Merge pull request #13 from kiela/master

Add ability to specify custom rake command