Implement webhook secret authentication

Tribtues to https://github.com/puppetlabs/puppet-webhooks/blob/dd33c30db1e595f05962da4b2f36810f165a4781/lib/puppet_labs/pull_request_app.rb#L64-83
Move webhook check to GithubWebhook class
Fix depreciation warnings
Add webmock
Mock relevant API calls
All specs are green
Fix last warning about secret_key_base (Rails 4)
Merge pull request #4 from ssaunier/check-github-signature

Check github webhook signature