Update default puppet version to 3.7.3
I had trouble using 'rake build' with the default 3.4 we'd been using
Remove gemfile.lock
See discussion on PR 148
Add gemfile.lock to gitignore
Merge pull request #148 from danieldreier/update_default_puppet_version

Update default puppet version to 3.7.3
Allow enabling directory environments

Directory environments are available in newer version of Puppet.

https://docs.puppetlabs.com/puppet/latest/reference/environments.html

This work allows for simple deprecation and migration to the new method
of achieving effectively the same dynamic branching strategy we've been
using for years.
Use inherited section
Provide doc on directory environments and add test
Set default on directory environments