Fix Travis CI Build

[sunjay]

Right now our builds are timing out because we're running all of our examples, even the ones that take forever.

Create a file in examples/ called ".test_examples" where each line is either a path relative to the examples directory (maybe starting with !), a line starting with #, or a blank line.

• All paths listed will be run during the CI build.
• All paths starting with ! will not be run.
• All paths in the examples directory must be listed in this file. That will be checked so CI fails if a file isn't listed.

The run all examples script (rewrite in Python) will be the one to read all of this and check those invariants. It should take a command line parameter for the number of groups and the group to run. Then we can split the examples into N groups and run those in parallel.

[sunjay]

Apparently public projects in Azure Pipelines get unlimited minutes. We can probably copy the config from turtle and use that in this repo to fix the build issues.

[sunjay]

Another idea is to create a single example that uses all the features of the ray tracer and run that instead of running every example. Note that we should still ensure that all examples are compiled.