Add 2.1.0 to .travis.yml

Ruby 2.1.0-preview1 was released today. Even though it'll still take
some time until a final version is released, we can already make sure
that rugged will be compliant from day one.
Merge pull request #267 from libgit2/arthur/add-2.1-to-travis

Add 2.1.0 to .travis.yml