Update default puppet version to 3.7.3

I had trouble using 'rake build' with the default 3.4 we'd been using
@actown I believe this resolves the issue you were having building the package, because I got the same error you showed me and it resolved when I used a newer puppet version. The gemfile was installing 3.4.
I'd be open to, I've just always thought that the best practice was to share gemfile.lock so that contributors would be using the exact same versions of gems. 

The travis-ci job deletes Gemfile.lock prior to installing, so it's testing the newest versions of gems. My thinking is that the travis tests get us  knowledge of whether newer gems break stuff, and the Gemfile.lock gets the convenience of working on similar versions most of the time.

I don't have strong feelings about it, though. The gems are just for developers so nothing breaks in production just because of gem versions.