remove matrix tag from travis env yaml

"matrix" doesn't belong in env param according to
http://docs.travis-ci.com/user/build-configuration/#The-Build-Matrix

Although it works as-is, having matrix under env prevents exclusions
from working.

.travis.yml

@@ -7,7 +7,6 @@ rvm:
   - 1.9.3
 script: bundle exec rake test
 env:
-  matrix:
   - PUPPET_VERSION="~> 2.7.0"
   - PUPPET_VERSION="~> 3.1.0"
   - PUPPET_VERSION="~> 3.2.0"