Fix requirements parser to support pip install -r requirements.txt

[rgerkin]

pip install -r requirements.txt requires pip installable lines, but pip install . uses the requirements parser to fill the install_requires portion of setup.py. There in no current way to write requirements.txt to work for both of these. The requirements parser must be changed to turn lines in requirements.txt like git+http://github.com/scidash/sciunit@dev#egg=sciunit-9999 into sciunit==9999 in install_requires. Then if the same parser puts the full line into dependency_requirements, at least pip install . can work with --process-dependency-requirements, while still allow pip install -r requirements.txt to work as well. This is a ridiculous hack, but Python package management is terrible.

[rgerkin]

Fixed with pip 18.1 and PEP 508 URL dependencies, as well as current version of setup.py.