Travis only runs a site build on master, not on PR

I just merged https://github.com/PyconUK/uk.python.org/pull/56 on the basis that Travis passed so it was probably fine

https://travis-ci.org/PyconUK/uk.python.org/builds/486397681 failed - and it looks like it only tries to actually do a site build on `master` and not for PRs, which leads to a rather false sense of security!