Consistently rewrite issue references to urls.greenkeeper.io

[boennemann]

We're still causing "Referenced by …" spam in some cases.
E.g. when a project uses repo/user#123 in their commit message.
We need to full render the commit message in html, rewrite the urls, and then add it to the commit.

webpack-contrib/sass-loader#267

[finnp]

Also referencing #299

[boennemann]

With the new Greenkeeper this issue is resolved https://github.com/integration/greenkeeper