Update plugin for use with guard > 2.9

This commit makes changes to the gemspec, specs, and plugin
implementation to allow the gem to be used with guard version >= 2.9.0.
`Guard::Guard` has been deprecated for awhile and was removed as of
guard 2.9, this replaces subclassing that with subclassing
`Guard::Plugin`.
Merge pull request #1 from sqm/update-for-newest-guard

Update plugin for use with guard > 2.9