Adding partial builds to Travis

[arcuru]

This switches the Travis builds so that it tests different subsets of the exercises depending on whether it is a PR build or a branch build, and on the changes made compared to the master branch. Basically, for a normal PR this will help lighten the load while it's being worked on, and we will run a full build against master once it is merged.

Some other tracks do this already, I've borrowed from the script used for Rust but heavily modified it.

If it's a branch build we run everything, as we currently do.
If it's a PR build that changes no exercises, we run nothing.
If it's a PR build that changes 1 or more exercises, we only run the changed exercises.
If it's a PR build that changes the root CMakeLists.txt file, we run everything to make sure that works. [1]

I'll be kicking off builds on Travis to test all the paths before this is ready to be merged.

[1] - This also changes the CMakeLists.txt file so it reads the directories instead of taking in a list of exercises, so it doesn't need to be edited just to add an exercise. It'll only be changed if there are real changes to it.

[arcuru]

So this now has examples for all the different paths:

Building everything in a branch (from my fork): https://travis-ci.org/patricksjackson/xcpp/builds/519523367
PR - building nothing (first build here)
PR - successfully building subset (second build here)
PR - failure when building subset (third build here)
PR - Running everything when root CMake changed (last build here)

[arcuru]

I'm going ahead and merging this, at it will save us compilation time and help with #272.