Make subsurface.centos6.sh run on travis-ci

[probonopd]

subsurface.centos6.sh uses a CentOS 6.7 Live ISO as a buildroot. This has the advantage that the resulting AppImage can run on more systems than if compiled with the normal Ubuntu 12.04 provided by travis-ci.

We should try to make subsurface.centos6.sh run on travis-ci.
Unfortunately chroot needs root and needing root means that we cannot cache the ISO and other ingredients needed for building.
Possibly we could get around this by using proot or something similar that doesn't need root rights.
https://smspillaz.wordpress.com/2015/01/04/creating-mini-distribution-containers-with-fake-root-access-on-travis-ci/

But then, simply mounting the ISO also needs root... :-(
Hey, tmp space should not be an issue so we might even unpack the ISO...

If you attempt this, please do so in a separate fork or in a "centos6-on-travis" branch.

[probonopd]

It's working now using docker on travis-ci...