Switch vagrant-cachier cache scope to :machine

Seems that NFS is likely throwing errors when using NFS for caching, if there's another machine sharing already:
https://gist.github.com/patcon/ed8efe8950ee1919e711

Didn't test extensively, but was having to disable NFS previously. It occurred to me that perhaps it was sharing with multiple vm's, and changing the scope from `:box` to `:machine` did suppress the error.

Any objection to changing it to smooth out the experience?