Add Ruby 2.0, 2.1, and 2.2. Remove 1.8.
Add Ruby 2.0, 2.1, and 2.2. Remove 1.8.

Add some confidence that the tests are working against newer versions of
Ruby. Even Ruby 1.9.3 has reached end of life at this point. Removes
1.8.7 from the list as support for it is already broken (see #57).

reinh/statsd