Add Ruby 1.9 on Linux and macOS

* 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.

ruby-builder-versions.js

@@ -1,6 +1,7 @@
 export function getVersions(platform) {
   const versions = {
     "ruby": [
+      "1.9.3-p551",
       "2.0.0-p648",
       "2.1.9",
       "2.2.10",