travis: fix Qt 5.6 build

[florianjacob]

Qt 5.6 was renamed, now also stating Qt's patchlevel.

Let's see whether the build works now…

[florianjacob]

Damn, seems like under that there's another error:

 ./test -platform offscreen

/home/travis/.travis/job_stages: line 57:  5323 Segmentation fault      (core dumped) ./test -platform offscreen

I tried updating test/lib/Catch, but that does not seem to have any effect. Everything works fine for me locally with Qt 5.9. I added other versions to travis to investigate, and 5.6, 5.7 and 5.8 fail, and 5.9 works on travis just fine as well.

@seanchas116 any idea what could cause this, or how to progress? Or do you happen to have older Qt versions available locally to reproduce?