Update TravisCI config to use Redis & Add Mocks

[elithrar]

Tests should be able to run, in part, without a running Redis instance via mocks.

• [x] Support configuration of Redis address for tests via env vars
• [x] Configure Travis to install Redis into the container & run tests against it
• [ ] Split tests into separate functions (unit tests vs. integration tests)
• [ ] Make integration tests optional / skippable when Redis not available via the -short flag.