fix (travis) Gemfile uses PUPPET_VERSION, our env should too
fix (travis) Gemfile uses PUPPET_VERSION, our env should too

after aligning our .dotfiles to those of puppetlabs, our Gemfile and our
.travis.yml's defaults are now a bit out of whack: they use different
environment variables for puppet's version. This pr aligns them.

dhoppe/modulesync_config,voxpupuli/modulesync_config,voxpupuli/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,puppet-community/modulesync_config,voxpupuli/modulesync_config,dhoppe/modulesync_config,dhoppe/modulesync_config