Update Guard::Resque to work with Guard versions after 2.9.0

The `Guard::Resque` plugin was inheriting from `Guard::Guard` which was
deprecated and then later removed in guard version 2.9.0. This commit
adjusts the plugin to inherit from `Guard::Plugin` to allow it to work
with versions of guard after 2.9.0.
Up required version of Guard to 2.0

This commit boosts the version of Guard that is required to at least 2.0
since that was the version that introduced `Guard::Plugin`. This also
adjusts the syntax in the specs to be compatible with rspec 2 instead of
only working with rspec 3.
Merge pull request #16 from sqm/update-for-newest-guard

Update Guard::Resque to work with Guard versions after 2.9.0