Upgrade xdot

Is there any specific reason that xdot is embedded internally?

* Current internal version is 0.4
* The [history of xdot](https://github.com/plasmodic/ecto/commits/master/python/ecto/xdot.py) in this repo seems to indicate it was just added without modifications.
* 0.4 is broken on trusty when calling `ecto.view_plasm(plasm)`, (see error report below)
* Trusty has 0.5 xdot as a deb
* Added xdot as a rosdep in [this pull request](https://github.com/ros/rosdistro/pull/4977)

This seems to work well.