Move to using a more recent build for the default devstack.

The dogwood devstack was not compatible with some changes made after dogwood was cut.

vagrant/release/devstack/Vagrantfile

@@ -75,7 +75,7 @@ openedx_releases = {
   # },
 }
 openedx_releases.default = {
-  :name => "dogwood-devstack-rc2", :file => "20151221-dogwood-devstack-rc2.box",
+  :name => "devstack-nightly-20160125", :file => "20160125-nightly-devstack.box",
 }
 rel = ENV['OPENEDX_RELEASE']