Travis-CI: only support Qt 5.4 on Linux for now.
Travis-CI: only support Qt 5.4 on Linux for now.

Indeed, brew is still with Qt 5.3.2, which breaks our build…

mirams/opencor,mirams/opencor,mirams/opencor,Fairly/opencor,mirams/opencor,Fairly/opencor,mirams/opencor,Fairly/opencor,Fairly/opencor,Fairly/opencor