PUPPET_VERSION has been renamed to PUPPET_GEM_VERSION but forgotten in CONTRIBUTING.md

Author: aerostitch

skeleton/CONTRIBUTING.md

@@ -30,7 +30,7 @@ By default the tests use a baseline version of Puppet.
 If you have Ruby 2.x or want a specific version of Puppet,
 you must set an environment variable such as:
 
-    export PUPPET_VERSION="~> 3.2.0"
+    export PUPPET_GEM_VERSION="~> 3.2.0"
 
 Install the dependencies like so...