Travis: Install and use GCC 5
Travis: Install and use GCC 5

Travis' newest supported distribution is Ubuntu 14.04 LTS (not joking) but we
can use a more current compiler. To match internal infrastructure we choose GCC
5.x which is the default in Ubuntu 16.04 LTS

Buchhold/QLever,Buchhold/QLever,Buchhold/QLever,Buchhold/QLever,Buchhold/QLever