Use environment variable instead of string for API key

After working with the tests for awhile, I got a little bored with
remembering to remove my API key. Using an environment variable
makes it easier to make updates to the tests and makes sure no
one accidentally commits their key.

I also considered using a config file but there was too much
overhead.
Updated to reflect testing changes
Merge pull request #31 from mattcan/env-var

Use environment variable instead of string for API key in tests