Use vagrant from master for development.

Gemfile

@@ -11,5 +11,5 @@ group :development do
   # We depend on Vagrant for development, but we don't add it as a
   # gem dependency because we expect to be installed within the
   # Vagrant environment itself using `vagrant plugin`.
-  gem "vagrant", :git => "git://github.com/mitchellh/vagrant.git", :tag => "V1.5.4"
+  gem "vagrant", :git => "git://github.com/mitchellh/vagrant.git",
 end