Require `cache.scope` to be set

[fgrehm]

Fork of #13 (comment)

what about requiring users to set the scope in order for the plugin to work and add a note to the readme about setting defaults from ~/.vagrant.d/Vagrantfile? this will make them think about their caching scheme before going crazy and caching all the things :)

[fgrehm]

0.6.0 will be the "deprecations release" (vagrant < 1.4, enable_nfs...) so it is the perfect time to bring this one in and I'll be working on it

[fgrehm]

This is done in git