Don't use pinned dependencies

Resolves #101

`requirements.txt` can continue to pin dependencies for the test suite, but the library itself only requires minimum versions. Only require test dependencies for testing, not for live installations.

References: 
* https://caremad.io/blog/setup-vs-requirement/
* https://pythonhosted.org/setuptools/setuptools.html#new-and-changed-setup-keywords