Add Ruby 1.8.7 compatibility

Many gems that depend on gems that depend on `devtools` (and therefore `yardstick`) are still tested against Ruby 1.8.7. If `yardstick` itself is not Ruby 1.8.7-compatible, it is impossible to test those gems for compatibility.

For example, the [`twitter` gem](https://github.com/sferik/twitter), which needs to maintain Ruby 1.8.7 compatibility, depends on `equalizer`, which [broke Ruby 1.8.7 compatibility in between versions 0.0.7 and 0.0.8](https://github.com/dkubb/equalizer/pull/12). It is currently impossible for `equalizer` to test itself against Ruby 1.8.7 because it depends on `devtools`.

I am well aware that Ruby 1.8.7, Ruby Enterprise Edition, and Kiji are no longer supported by their respective core teams, however there are still many companies running legacy applications on these versions in production. Gem maintainers that want to continue to support these applications need to test their applications on Ruby 1.8.7 and this is currently not possible if such gems depend on any gems that use `devtools`. These relatively benign changes will fix this real-world problem.

This pull request is co-dependent on https://github.com/rom-rb/devtools/pull/47.