Set up Travis so it runs `make` with both clang and gcc

- Fixes #38.
- You'll still need to sign in to Travis and enable this repository to finish setting up Travis. This PR only includes the configuration file and build status badge (which isn't activated yet).
- For now, this just runs `make` with both clang and gcc. The build will fail if there is an issue with either compiler. I'm assuming this is what we want.
- I'm not running `make test` on CI because it's currently broken. #30 should fix this, then just add `&& make test` to the end of the script.
- We still don't have unit tests. We probably don't need them yet, but that would be a good idea for the future.