Prevent apt cacher bucket from being installed in case NFS is disabled

lib/vagrant-cachier/bucket/apt_cacher.rb

@@ -12,6 +12,7 @@ module VagrantPlugins
           guest   = machine.guest
 
           if guest.capability?(:apt_cacher_dir)
+            if machine.config.cache.enable_nfs
               guest_path = guest.capability(:apt_cacher_dir)
 
               @env[:cache_dirs] << guest_path
@@ -24,6 +25,9 @@ module VagrantPlugins
                   comm.sudo("ln -s /tmp/vagrant-cache/#{@name} #{guest_path}")
                 end
               end
+            else
+              @env[:ui].info I18n.t('vagrant_cachier.nfs_required', bucket: 'apt-cacher')
+            end
           else
             @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'apt-cacher')
           end

locales/en.yml

@@ -6,6 +6,9 @@ en:
       Skipping %{bucket} cache bucket as the guest machine does not support it
     unknown_cache_scope: |-
       Unknown cache scope '%{cache_scope}' (allowed scopes: %{allowed})
+    nfs_required: |-
+      The '%{bucket}' cache bucket requires NFS to be enabled, please add
+      `config.cache.enable_nfs = true` to your Vagrantfile.
     will_fix_machine_cache_dir: |-
       A vagrant-cachier provider specific cache dir was found under
       '%{current_path}' and it will be moved to