Implement generic bucket

This bucket must be explicitly enabled and may be useful for users
wanting a generic caching mechanism that they can use directly instead
of via some predefined packaging system.

For instance, this enables one to implement a cache for packages
downloaded via wget or curl.
Update bundle
Up version to 0.7.0 as we will add a new feature to the plugin
Merge branch 'generic-bucket' of https://github.com/gnustavo/vagrant-cachier
Minor tweaks to generic bucket docs [GH-94]
Skip bucket auto detection in case the bucket can't be auto detected [GH-94]
Add missing `end` to generic bucket and remove `capability` method [GH-94]
Some more tweaking to generic bucket docs [GH-94]
Update changelog
Tweak Gemfile for vagrant 1.5+