Specify version number for packages in test-requirements.txt

As you may have noticed, the last month or so of Travis builds have been failing:

```
FAILED (errors=3)
make: *** [functional] Error 1

FAILED (failures=5)
make: *** [unit] Error 1
```

Specifying the version number for the packages in the requirements.txt makes the Travis builds pass again.
Specify version number for urllib3 in requirements.txt

Should fix `ERROR: HTTPretty should be able to record calls` and `ERROR: HTTPretty should be able to record calls`.
Merge pull request #159 from luqmaan/travis-build-fixes

Make Travis builds work again