Exclude tests for Puppet < 3.2 against ruby-head

ruby-head is currently 2.x, which Puppet only began supporting at version
3.2: http://projects.puppetlabs.com/issues/18494

This should make the "allowed failures" builds green once again.

.travis.yml

@@ -23,5 +23,9 @@ matrix:
       env: PUPPET_GEM_VERSION="~> 2.6.0"
     - rvm: ruby-head
       env: PUPPET_GEM_VERSION="~> 2.6.0"
+    - rvm: ruby-head
+      env: PUPPET_GEM_VERSION="~> 3.0.0"
+    - rvm: ruby-head
+      env: PUPPET_GEM_VERSION="~> 3.1.0"
 notifications:
   email: false