Puppet 4 is not longer an acceptable failure in CI

With the recent changes for considering EL 7 nodes, this module will be
considered to be acceptable to run in Puppet 4. In this case, we want
spec failures with Puppet 4 testing to result in failures in the TravisCI
system.

This change removes Puppet 4 as an "acceptable failure" in the TravisCI
system.

.travis.yml

@@ -11,9 +11,7 @@ matrix:
   allow_failures:
     - rvm: 1.8.7
     - rvm: 2.0.0
-      env: PUPPET_VERSION="2.7"
     - env: PUPPET_VERSION="2.7"
-    - env: PUPPET_VERSION="4.0"
 
 before_install:
   - 'gem install bundler'