Added support for Travis-CI and main ruby-versions

I added also ruby-head to the ruby versions to be tested, as it will make us aware of possible problems with upcoming versions of ruby.

If you want some ruby-versions (e.g. ruby-head not failing your travis build) add the following lines to .travis.yaml
matrix:
+  allow_failures:
+    - rvm: ruby-head

Replaces https://github.com/jenkinsci/puppet-jenkins/pull/34

See also Travis https://travis-ci.org/ngiger/puppet-jenkins/builds/11764964