Restore Ruby 1.8.7 compatibility

It appears that Ruby 1.8.7 compatibility was removed in version 1.99.1, in violation of [Semantic Versioning](http://semver.org/). If you want to drop support for Ruby 1.8.7, please release `rainbow` version 2.0.0, but before you do that, please restore compatibility by merging this patch and releasing version 1.99.2.

Thanks.