Build against latest Ruby 2.1 on Travis

In contrast to prior versions, we can specify the latest Ruby 2.1
as `2.1` due to the new versioning policy.

https://github.com/travis-ci/travis-ci/issues/2220#issuecomment-42263699
Merge pull request #153 from yujinakayama/travis-latest-ruby-2.1

Build against latest Ruby 2.1 on Travis