Add Ruby 1.8.7 compatibility

Many gems that depend on gems that depend on `devtools` are still tested against Ruby 1.8.7. If `devtools` itself is not Ruby 1.8.7-compatible, it is impossible to test those gems for compatibility. This patch adds Ruby 1.8.7 compatibility.

This pull request depends on https://github.com/dkubb/yardstick/pull/16.