Provide builds compatible with older distros

[TheAssassin]

For convenience and to keep required maintenance efforts low, we build on Travis CI on the latest supported Ubuntu LTS distro.

Unfortunately, there's older distros which this distro (as of writing, Ubuntu xenial) is incompatible with ABI wise. This means that our released AppImages can't easily be run on those platforms.
Normally, they can be built fine, since we use only C++11, which is supported in e.g., CentOS 7 out of the box.

We should consider offering special builds for older distros. A suitable target is CentOS 7, but if possible CentOS 6 might be not much more work but increases compatibility even further.

CC @jonaski.

[jonaski]

I just ended up compiling linuxdeploy myself, so no big issue for me atm.

[TheAssassin]

Surely not a big priority, but IMO the whole linuxdeploy ecosystem should be released in a way that is compatible to the old distros. Unfortunately, it's a lot of work and only helps a few people with special demands, therefore the priority is low.

We've limited ourselves originally to using C++11 to at least allow people to easily compile our software there, you demonstrate that this works as intended. Using C++11 is at least not as much of a PITA as using older releases or even C.