Switch to the master branch for urdfdom (#1190)

In ros/urdfdom#158 and ros/urdfdom#161,
we merged the ros2 branch onto the master branch so there would be single,
unified release branch for all of urdfdom.  Now that that is done, we should switch
to using the master branch of urdfdom.

ros2.repos

@@ -194,7 +194,7 @@ repositories:
   ros/urdfdom:
     type: git
     url: https://github.com/ros/urdfdom.git
-    version: ros2
+    version: master
   ros/urdfdom_headers:
     type: git
     url: https://github.com/ros/urdfdom_headers.git