Lower version numbers of some third-party packages

We haven't yet decided how we want to do versioning for packages that
mostly just wrap non-Meteor code that has its own version numbers. We
might stick to totally-unrelated version numbers (and maybe add a
"wrapped version" field that gets displayed in the upgrade/downgrade
messages?), or change to matching upstream versions (with techniques for
dealing with changes to packaging, a la debian_revision), or something
different.

But since changing to match upstream versions is a possibility, let's
make sure that that operation won't be viewed as a "downgrade" by
updating the wrapped packages whose upstream versions are 0.*.

packages/backbone/package.js

@@ -1,6 +1,6 @@
 Package.describe({
   summary: "A minimalist client-side MVC framework",
-  version: "1.0.0"
+  version: "0.9.2"
 });
 
 Package.on_use(function (api) {

packages/showdown/package.js

@@ -4,7 +4,7 @@
 
 Package.describe({
   summary: "Markdown-to-HTML processor",
-  version: "1.0.0"
+  version: "0.3.1"
 });
 
 Package.on_use(function (api) {

packages/stylus/package.js

@@ -1,6 +1,6 @@
 Package.describe({
   summary: 'Expressive, dynamic, robust CSS',
-  version: "1.0.3"
+  version: "0.46.3"
 });
 
 Package._transitional_registerBuildPlugin({