use eventmachine at github master

because eventmachine 1.0.3 cannot compile on ruby 2.2.0

ttps://github.com/eventmachine/eventmachine/commit/c95f1e11c2c0f0e23cd9d4ac2a9c7a75afe54710
change Gemfile to sorted style

* sort gems by ascii
* prefer double quotation instead ob single quotaton
* use sub-group style for development/test group gems

cf. https://gist.github.com/kakutani/43b9f42197ab002fcdf8
remove unused sub-group