Run tests against puppet 3.7

Puppet 3.7 [deprecates a number of language features](https://docs.puppetlabs.com/puppet/3.7/reference/deprecated_language.html) in preparation for the future parser to become the default in 4.0. To ensure that puppet-syntax is a usable tool for detecting use of these features, we should test against Puppet 3.7.

This relates to PR #28.