Travis CI activated

I’ve re-activated Travis CI for LT. We had used it more than a year ago but had to turn it off due to memory problems. I hope it will work better this time:

Travis will run all unit tests automatically on every commit/push, and also for every pull request. Thus, we’ll get much faster feedback than we did, because we could run the unit tests only once per day. Still, any developer is expected to run the tests locally on their machine before pushing a change.