Don't skip so many tests

fixes #286 for now

It all boils down to `skip-travis` and testing [measures](https://github.com/ufo-kit/concert/blob/master/concert/tests/unit/test_measures.py#L11). When I comment out that one line everything works. I think this goes pretty deep in nose itself.
I was thinking about threading problems, but when one specifies the `skip-travis` attribute the tests are *not* executed, so how can it fail then? Maybe the plugin somehow forgets to delete the attributed tests from some nose's "to be done" queue or something.
Anyway, the tests are 4x slower now and there is absolutely no guarantee we won't have the same problem in the future. But at least the tests pass and we see it is not actually connected to our code (or in a very strange way). Regarding the test-skipping in general, I can live with 20s execution rather than fixing already merged stuff as in #282. We should maybe only skip stuff which cannot be executed by Travis.