trusty + clang + C++14 is non-operational and Travis is so bad that they're
only concerned with whitelisting packages piecemeal instead of providing a
usable product, or a modern LTS like xenial
* .travis.yml: Use an explicitly defined build matrix.
* Attempt to use GCC on Mac.
* Add gcc4.8 to Travis deps.
* Remove gcc set from .travis.yml.
* .travis-deps: Install and set compiler to GCC only if we need it.
* Formatting
* Remove redundancy
* .travis.yml: source the deps script.