add Coveralls

* CoverallsでRailsアプリのテストのカバレッジ率を取得する手順メモ: http://qiita.com/knt45/items/58cacf895c0e5ad9228b
try fix an error on Travis-CI by hand generate

$ bundle install --without development --deployment
You are trying to install in deployment mode after changing
your Gemfile. Run `bundle install` elsewhere and add the
updated Gemfile.lock to version control.