Store the available Ruby versions in this repository

* That way it's always perfectly synchronized with the action. In
  practice querying another repository sometimes returned a stale copy.
* Avoids an extra HTTPS request.
* The axios dependency is no longer needed.
* Makes it possible to tweak the versions available according to the platform.

ruby-install-builder-versions.js

@@ -0,0 +1,25 @@
+export function getVersions(platform) {
+  const versions = {
+    "ruby": [
+      "2.3.0", "2.3.1", "2.3.2", "2.3.3", "2.3.4", "2.3.5", "2.3.6", "2.3.7", "2.3.8",
+      "2.4.0", "2.4.1", "2.4.2", "2.4.3", "2.4.4", "2.4.5", "2.4.6", "2.4.7", "2.4.9",
+      "2.5.0", "2.5.1", "2.5.2", "2.5.3", "2.5.4", "2.5.5", "2.5.6", "2.5.7",
+      "2.6.0", "2.6.1", "2.6.2", "2.6.3", "2.6.4", "2.6.5",
+      "2.7.0"
+    ],
+    "jruby": [
+      "9.2.9.0"
+    ],
+    "truffleruby": [
+      "19.3.0", "19.3.1"
+    ]
+  }
+
+  if (platform === 'ubuntu-18.04') {
+    versions['rubinius'] = [
+      "4.13"
+    ]
+  }
+
+  return versions
+}