Stop parsing attachment ids from attachment urls

See 2111294

Attachment ids are now available in string form from the server
response to `putAttachment`.

// FREEBIE

libtextsecure/api.js

@@ -391,7 +391,7 @@ var TextSecureServer = (function() {
                     data        : encryptedBin,
                     processData : false,
                 }).then(function() {
-                    return match[1];
+                    return response.idString;
                 }.bind(this));
             }.bind(this));
         },