Enable TravisCI for our master branch

Pull requests from forks aren't being tested by Travis. We have a theory
that this is because we've disabled builds for the master branch and there's
a hint in the docs that pull requests don't work the same as normal branch
builds: http://docs.travis-ci.com/user/pull-requests/#My-Pull-Request-isn't-being-build

> We rely on the merge commit that GitHub transparently creates between the
> changes in the source branch and the upstream branch the pull request is
> sent against.

The reason we originally disabled this branch is because it's also tested by
our in-house CI. However in reality it's not such a big deal for it to be
tested twice. At worst we tie up a Travis worker for a little longer than we
need.
Merge pull request #69 from alphagov/72271312-enable_travis_master_branch

[#72271312] Enable TravisCI for our master branch