Join env variable retrieval and default
Lookup both cases for wercker_url env var

In my Mac the environment variable WERCKER_URL was not
used when it was exported. When I added a upper case key
in the config.py, it worked. I added both checks (low and upper case).
Fix unit tests fail when wercker_url is set as env var
Adds ENV_KEY_WERCKER_URL and update unit tests

Adds ENV_KEY_WERCKER_URL to config.py and updates unit tests to use
that key instead of raw string value
staying away from port 42, it's now 42*42
Merge pull request #6 from wercker/origin/fix-wercker-url

Origin/fix wercker url