metadata.json: Fix URL to source repo

the `git://` URIs are deprecated, best practice is to reference to
https.

Author: bastelfreak

metadata.json

@@ -4,7 +4,7 @@
   "author": "puppetlabs",
   "summary": "Module for installing and managing docker",
   "license": "Apache-2.0",
-  "source": "git://github.com/puppetlabs/puppetlabs-docker",
+  "source": "https://github.com/puppetlabs/puppetlabs-docker",
   "project_page": "https://github.com/puppetlabs/puppetlabs-docker",
   "issues_url": "https://github.com/puppetlabs/puppetlabs-docker/issues",
   "dependencies": [