Support for extra synced_folder options

Added ability to pass extra synced_folder options through new
config.cache.sync_opts -configuration.
Update bundle
We'll drop support for Vagrant < 1.4, that means we should be bumping to 0.6.0
After 5 months I believe it is safe to get rid of this code
This is no longer needed
We'll probably forget to update these guys, better do it now :)
Update docs to reflect what's going to be implemented
Replace `enable_nfs` and `sync_opts` config with `synced_folder_opts`
Mark `config.enable_nfs` as deprecated
Mention `synced_folder_opts` on CHANGELOG
Lock vagrant to 1.4.3
Display a warning using Vagrant's UI in case enable_nfs is set
Display enable_nfs deprecation warning only once
When specifying NFS synced folders we need to use a symbol (fix GH-76)
Update CHANGELOG
Blow up if using vagrant < 1.4.0
Extract cachier's root bucket directory creation out to a separate action [GH-60]
:bomb: monkey patch on Builtin::Provision action \o/ (closes GH-60)
Clean up InstallBuckets action a bit
Improved logging