Add support for non-pinned requirements

[joeyespo]

This will solve the Error: Can only work with pinned requirements for now case and close out this TODO.

[jamesbroadhead]

^ this code-path should probably raise rather than fall-through to the next line.

Reading ./scripts/env/requirements.txt...
Error: Can only work with pinned requirements for now.
Traceback (most recent call last):
  File "/Users/jbo/venv/default/bin/upgrade-requirements", line 11, in <module>
    load_entry_point('upgrade-requirements==1.7.0', 'console_scripts', 'upgrade-requirements')()
  File "/Users/jbo/venv/default/lib/python3.7/site-packages/upgrade_requirements.py", line 58, in main
    name, version = requirement.split('==')
ValueError: not enough values to unpack (expected 2, got 1)