fix: Apply same optimisation to upload hook

We were again grabbing _every_ uploaded revision in order to be
able to check if you were trying to overwrite a revision. This is slow
when you have many revisions uploaded to s3.

Here we prefer our new `findRevision` method which uses the
`Prefix` to load information about _just_ the revision we're interested
in.

Author: Kelvin Luck

lib/s3.js

@@ -83,10 +83,9 @@ module.exports = CoreObject.extend({
       params.ContentEncoding = 'br';
     }
 
-    return this.fetchRevisions(options)
-      .then(function(revisions) {
-        var found = revisions.map(function(element) { return element.revision; }).indexOf(options.revisionKey);
-        if (found >= 0 && !allowOverwrite) {
+    return this.findRevision(options)
+      .then(function(found) {
+        if (found !== undefined && !allowOverwrite) {
           return RSVP.reject("REVISION ALREADY UPLOADED! (set `allowOverwrite: true` if you want to support overwriting revisions)");
         }
         return RSVP.resolve();