don't pin sinatra and webrick gems anymore

[kenyon]

Affected Puppet, Ruby, OS and module versions/distributions

• Puppet: 6.0
• Distribution: Debian 9.6
• Module version: master

What are you seeing

Now that WEBrick is in the Ruby standard library, and Puppet Platform 6.0 packages include Ruby 2.5.1, we shouldn't need to pin the webrick and sinatra versions anymore. So I believe these commits could just be reverted:

• 312e759
• d36db9a

[kenyon]

Alternatively, setting the defaults for r10k::webhook::package::sinatra_version and r10k::webhook::package::webrick_version to 'installed' would work too.