Only attempt to install GHE repos if requested

This install script will preduminantly be used by 3rd parties that do
not have access to our Github Enterprise.
Simplify bootstrap-vagrant

- Simplify how gems are installed
- Make librarian-puppet install destructive so that broken
  cached modules are removed and reinstalled rather than staying
  broken.
Merge pull request #30 from alphagov/simplify-bootstrap

Simplify bootstrap