Make Gemfile.lock agree about librarian-puppet ver

The Gemfile and Gemfile.lock disagree about what version should be pinned.
Which doesn't seem to upset Bundler until you add a new entry to Gemfile:
```
Resolving dependencies...
You have requested:
  librarian-puppet = 0.9.14

The bundle currently has librarian-puppet locked at 0.9.13.
```
Use puppet-syntax gem

This code has since been improved and split out to a gem. It now includes
syntax checks for YAML in `hieradata/`.
Merge pull request #343 from alphagov/puppet_syntax_gem

Use puppet-syntax gem