* On Windows there are no 64-bit builds of Ruby 1.9.
* Fix floatVersion() for head versions.
* Use floatVersion() in more cases instead of regexps.
* Rely on ~/.gemrc to not install gem docs.
* This is a special case, because TruffleRuby needs an extra system
dependency that other Rubies do not, and GitHub runners do not have
these packages installed by default, while TravisCI has them.