Ensure right Puppet version in place for Travis

Keep PUPPET_VERSION for any existing uses, use PUPPET_GEM_VERSION for the
version spec that is supplied via .travis.yml.

Gemfile

@@ -2,6 +2,8 @@ source "http://rubygems.org"
 
 if ENV.key?('PUPPET_VERSION')
   puppetversion = "= #{ENV['PUPPET_VERSION']}"
+elsif ENV.key?('PUPPET_GEM_VERSION')
+  puppetversion = ENV['PUPPET_GEM_VERSION']
 else
   puppetversion = ['~> 2.7']
 end