v1.4.0

Enhancements:

• #28 Replace calls to exit for sys.exit
• #27 If reviews are disabled, do not look in the review history
• #26 Update documentation
• #21 Option to disable PR reviews being posted
• #25 Migrate Travis to GitHub actions
• #24 Fix all flake8 warnings
• #23 Improve exception handling
• #22 Add support for defining invalid labels