Only run Rake tests on branches

When pushing straight to master, via Micropub, any validation of the content should be done before it reaches GitLab, and therefore I should have confidence it will not break the site's build, or output.

(If it does, that's an issue that needs to be resolved within the Micropub server. Additionally, if this did break master, it'd be very difficult for me to resolve it from i.e. mobile.)

Therefore, master builds are slowed down by ~4 minutes for no added benefit, which means publishing new content via Micropub is slower, and reduces the real-time feel of my site and Social Web interactions.

Additionally when pushing via the CLI, I should be checking it myself - and if not, that's on me.

However, on branches I am often adding new articles, or making larger functional changes to the site which definitely should be validated with HTMLProofer to ensure that the content is correct, and any changes to the site do not break my existing functional expectations.

Therefore, we can remove running HTMLProofer, or any other Ruby checks, as part of the build on master, and instead verify these only on branches.