Remove devtools as a development dependency

Devtools are not currently compatible with Ruby 1.8.7, which this
library will support until version 1.0.0 is released. See discussion at
https://github.com/rom-rb/devtools/pull/47.
Remove guard configuration
Run specs on REE, MRI 1.8.7, and JRuby in 1.8-mode
Add back yardstick for documentation coverage
Add back RuboCop
Add back Reek