Modified test framework for sentinel & travis updates

This should make it possible to test normal and sentinel functionality in travis, and shouldn't affect normal testing functionality (although it _does_ change the default ```TEST_LEDGE_REDIS_SOCKET```, but I can't think of a good way around that).

Usage:

```make test TEST_LEDGE_REDIS_SOCKET=/tmp/redis.sock``` will function as the current Makefile does, simply running through the ledge tests on any currently running redis instance on the default port. 

```make test_all``` will run the full test suite, including the sentinel tests, and will spawn the relevant number / configuration of redis and sentinel processes to run through the two sets of tests. For this to work properly it means that there has to be no process running on the default redis port.