Switch to using master branch for daily builds

[matthew-brett]

I propose to give up on the daily branch, which I had previously been using for daily builds, and use the master branch instead.

Then doing a release is the natural process of branching off master, and committing.

Any build on master uploads to the pre-release Rackspace container, but builds on any other branch upload to the wheels container from which we usually download / upload to PyPI.

The advantage is, it's more obvious. For example, I had edited the daily branch to fix the pkgconfig / six dependency, but forgot when you asked me about the related error you were getting, on master.

OK with y'all?

[tacaswell]

👍