Set up Travis CI

[brettcannon]

With caching turn on in the .travis.yml file, the base set of benchmarks should complete for a single run before Travis times out and kills the VM. That would be enough to make sure simple things like the django -> django_template rename breakage don't occur again.

[vstinner]

For tests, I'm using --debug-single-sample command line option: it asks to only run each benchmark once using a single worker process (or maybe even no worker process at all, depending on the benchmark is run).

• "python3 bench.py run --debug-single-sample" => 14 seconds (10 benchmarks)
• "python3 bench.py run --debug-single-sample -b all" => 28 seconds (44 benchmarks)

The purpose of tests is to check that running benchmarks work, not to check the results.

[vstinner]

Done: see .travis.yaml.