Activate osx builds on travis, at low priority

We want to activate osx builds on Travis CI. But they can be very slow, so we need to customise the settings.

If possible, we want osx builds to behave as follows:

• if there is a pending build, and more commits are pushed to the branch, cancel the pending build and re-queue the latest commits
• let travis show builds as "complete" if osx takes a long time, but still show osx failures eventually