Adds travis for linux

[jlaura]

Copy from the SensorUtils repo.

[jessemapel]

Does this bring in the CSM dependency?

[jlaura]

The update does. Check the change to the CMakeLists.txt to see if that looks acceptable to hint (would be mainly used by Travis and Appveyor). I need to see why this PR is not travis testing though. I bet because no .travis.yml is present in the root.

[jlaura]

This is getting to be a larger PR than I like. Apologies.

Adds the following:

• Travis Stub (this PR needs to get merged to have travis fire - this PR does not necessarily have a fully passing travis build. Local builds are working on OS X and Ubuntu 18.04)
• Addition of GTest as a submodule
• Modification of root CMakeLists.txt to build GTest at compile time
• Modification of tests/CMakeLists.txt to turn off GTest discovery. Since we build GTest, we do not need to discover the location.

This PR does not touch appveyor, so that failure is a false positive.