Add phony target to Makefile for test and install

Recently, I added a top-level directory called `test`, which unfortunately can cause `make` to think that the `test` target is up-to-date.

@stephensanchez Please review.  I'd like to get this merged soon as I've seen [at least one Travis build](https://travis-ci.org/edx/edx-ora2/builds/30680449) pass without actually running the tests.