Add optional support for vagrant-cachier plugin.

docs/fdroid.texi

@@ -116,6 +116,8 @@ Ruby (debian packages ruby and rubygems)
 Vagrant (unpackaged) Be sure to use 1.3.x because 1.4.x is completely broken
 (at the time of writing, the forthcoming 1.4.3 might work)
 @item
+vagrant-cachier plugin (unpackaged): `vagrant plugin install vagrant-cachier`
+@item
 Paramiko (debian package python-paramiko)
 @item
 Imaging (debian package python-imaging)

makebuildserver

@@ -146,6 +146,10 @@ for f, src, shasum in cachefiles:
 vagrantfile = """
 Vagrant::Config.run do |config|
 
+  if Vagrant.has_plugin?("vagrant-cachier")
+    config.cache.scope = :box
+  end
+
   config.vm.box = "{0}"
   config.vm.box_url = "{1}"