Add g++-6 builds to travis-ci

The current linux/g++ testing (using a backported g++-4.8 on a
4-year-old Ubuntu) is quite ancient.  It's good as a baseline level of
support, but it means we aren't testing g++'s C++14 support at all
(which is why #334 happened).

This commit adds a docker-based travis-ci build using the debian
"testing" distribution, which will give us both another test system
(with different versions of build tools), while, more importantly, also
adding a build and test run using g++ in C++14 mode.