change Gemfile to sorted style

* sort gems by ascii
* use sub-group style for development/test group gems

cf. https://gist.github.com/kakutani/43b9f42197ab002fcdf8
remove unused sub-group
fix Gemfile.lock
Merge pull request #203 from onk/feature/idobata_style_gemfile

change Gemfile to sorted style
[ci skip]