Fix spelling of 'max_issue_threshold'

[lorarjohns]

Without this fix, args['max_issue_threshold'] will throw a KeyError in the following code block:

if (args['max_issue_threshold'] is not None
            and args['max_issue_threshold'] <= total_issues):
        print(("Found {issues} issues, but only "
               "{max} permitted!").format(
                   issues=total_issues,
                   max=args['max_issue_threshold']))
        sys.exit(1)

Also added a requirements-dev.txt for pytest requirements.

[troycomi]

Thank you so much for catching that! I thought I had changed all those typos but it looks like the tests weren't running what I thought.

I made a separate pull request (#10) to also correct some issues with testing to improve coverage. I took the recommendation for developer requirements and placed them into the setup.cfg, per this guide. Closing this, but feel free to contribute in the future.