OpenBLAS

Travis: Simplify configuration using Build Stages and APT addon

Using APT addon has nice side-effect - you don't need sudo anymore, so
it can run on Travis containers-based infrastructure that is much faster
than their VMs infrastructure (used when sudo is needed).

You've been still running on Ubuntu Presty builders, but new default is
Trusty. Thus I've explicitly set `dist: presty` to let it stay on
Presty, to not change build environment by this commit.