Potential Bugfix: Attempt to run builds serially

Master is currently failing tests on Travis. Specifically, this test is failing... Sometimes.

```
1) Failure:
WorkerTest#test_worker_ueses_one_conn [/home/travis/build/ryandotsmith/queue_classic/test/worker_test.rb:129]:
Multiple connections found -- are there open connections to postgres://postgres@localhost/queue_classic_test in other terminals?.
Expected: 1
  Actual: 2
```

I can't reproduce locally on my machine. I suspect that since we tell travis to run the test on 3 different rubies that there is some overlapping tests which would cause more database connections than we expect.