Grunt and Gemfile changes

Summary:
- Fix grunt css:clean task
- Move Gemfile to top level
- Move bower_components ignore to modules/UshahidiUI/.gitignore
- Run compass via bundle exec (this should mean we run a known version of compass.)

Test Plan:
- Run 'bundle install' at the root
- Run grunt css:clean
- Run grunt compass

Reviewers: shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D148