Revert "Remove devtools as a development dependency"

This reverts commit 79bc7203926b5093b7c82ffe44a0e8248c06d8b2.
Add guard configuration
Drop support for Ruby 1.8.7 and 1.9.2
Revert "Restore Ruby 1.8.7 compatibility"

This reverts commit e29af783bbc4b162f067342c6da0506fe3e84e0e.
Revert "Fix specs on Ruby 1.8.7"

This reverts commit 353adb32fb227a8f8b5b643e319c578de026457d.