Update Ruby to 2.1.5.

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/
Update Bundler to 1.7.6.
Merge pull request #5 from yugui/feature/update

Update Ruby and Bundler to the latest versions.
Build deployment environment but not development environment.

This includes:
* Run bundler with deployment mode
* Sepcify -E production when rackup.