Upgrade HtmlUnit to v2.70.0

In this release, I upgraded to the latest release of HtmlUnitDriver (v2.70.0). The dependencies of this new release changed slightly, which necessitated a few minor tweaks to HtmlUnitPlugin.

This is the first official release of Selenium Foundation after I removed the parallel Maven project files. These are no longer useful, as I finally added the last missing capabilities to the Gradle project files for Selenium Foundation. I've updated the documentation to reflect the removal of Maven support.