add force_forge_versions source parameter to g10k config

[xorpaul]

you can enforce version numbers of Forge modules in your Puppetfiles instead of :latest or :present by adding force_forge_versions: true to the g10k config in the specific resource

---
:cachedir: '/tmp/g10k'

sources:
  example:
    remote: 'https://github.com/xorpaul/g10k-environment.git'
    basedir: '/tmp/example/'
    force_forge_versions: true

If g10k then encounters :latest or :present for a Forge module it errors out with:

2016/11/15 18:45:38 Error: Found present setting for forge module in /tmp/example/example_benchmark/Puppetfile for module puppetlabs/concat line: mod 'puppetlabs/concat' and force_forge_versions is set to true! Please specify a version (e.g. '2.3.0')