Test with Ruby 2.1.0 on Travis and add it to the supported Rubies

## Changes
- Test with Ruby 2.1.0 on Travis.
- Add Ruby 2.1.0 to the supported Rubies in the readme.

## Related posts
- [Ruby 2.1.0 is released](https://www.ruby-lang.org/en/news/2013/12/25/ruby-2-1-0-is-released/)
- [Support for Ruby version 1.9.3 will end on February 23, 2015.](https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015/)