Add Ruby 1.8.7 compatibility

I’m interested in using `adamantium` in the `twitter` gem, however, this will require Ruby 1.8.7 support, at least for the next 6 months. I am happy to do all the maintenance work myself. I’m also okay with Ruby 1.8.7 support being dropped when version 1.0.0 is released, just as we’re doing with `equalizer` and `memoizable`.