Travis only runs a site build on master, not on PR #58

I just merged UKPythonAssociation/uk.python.org#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!