Remove .rvmrc and update dependency versions in gemspec

This commit updates the dependencies in the gemspec to be the current
versions so we can start updating the functionality of the plugin. This
also removes the .rvmrc.
Update Guard::JslintOnRails to inherit from Guard::Plugin

`Guard::Guard` has been deprecated and was removed from guard as of
version 2.9. This commit makes the necessary upgrade steps, switching
from inheriting from `Guard::Guard` to inheriting from `Guard::Plugin`
and modifying the initilizer.