Run demo app tests on travis builds

[drewhannay]

Now that we've added a sample app (in #7), we can write tests for the project. We should be running those tests on every travis build. This will require a way to install the Test Butler app on the emulator in travis before running tests.

[CristianGM]

👍 And it will provide an example for the users of the library on how to use it for their Travis builds.