Bok-choy

* Bug fix to XSS vulnerability auditing feature

* Implement feature for XSS (cross-site scripting) vulnerability auditing
* Added `scroll_to_element(element_selector)`
* Make browser instantiation more resilient to transient webdriver failures
* Disable default promo tabs on firefox profile
* Updates for contributors (i.e. we use Slack now, not IRC)
* Various improvements for how the bok-choy repo itself is tested
* Python version compatibility improvements

* Skipped this release version due to infrastructure udpate (i.e., not releasing via Travis)

* No changes. Infrastructure updates.

* Ability to release to pypi via Travis

* Improved check for custom rules file format [a11y rules]
* Use docker for Travis builds, some fixed linting
* Capability for custom firefox path using the SELENIUM_FIREFOX_PATH environment variable