fix: Find the precise item we asked for

I guess there is the possibility that we could have retrieved
information about a revision with a `Key` which *starts with*
the `Prefix` we passed (rather than the entire filename
being the prefix we passed).

It's pretty easy to guard against this edge case so let's do
so...

Author: Kelvin Luck

lib/s3.js

@@ -149,7 +149,8 @@ module.exports = CoreObject.extend({
     var prefix         = options.prefix;
     var revisionPrefix = joinUriSegments(prefix, options.filePattern + ":" + options.revisionKey);
 
-    return listObjects({ Bucket: bucket, Prefix: revisionPrefix }).then((response) => response.Contents[0]);
+    return listObjects({ Bucket: bucket, Prefix: revisionPrefix })
+      .then((response) => response.Contents.find((element) => element.Key === revisionPrefix));
   },
 
   fetchRevisions: function(options) {