Add libx11-dev to the list of packages to install.

A number of packages are currently failing to build because the [graphics][graphics] package is missing.  For example:

* [cairo2][cairo2] (and [here][cairo22])
* [eliom][eliom] (and [here][eliom2])
* [litiom][litiom] (and [here][litiom2])
* [mesh][mesh] (and [here][mesh2])
* [ocamlviz][ocamlviz] (and [here][ocamlviz2])
* [async_graphics][async_graphics] (and [here][async_graphics2])

This pull request adds `libx11-dev` to the DockerScript so that the graphics packages builds.

[graphics]: https://github.com/ocaml/ocaml/tree/trunk/otherlibs/graph

[async_graphics]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/async_graphics#L6631
[async_graphics2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/async_graphics#L6684
[cairo2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/cairo2#L403
[cairo22]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/cairo2#L432
[camlimages2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/camlimages#L951
[eliom]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/eliom#L8297
[eliom2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/eliom#L8893
[litiom]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/litiom#L8306
[litiom2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/litiom#L8903
[mesh]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/mesh#L174
[mesh2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/mesh#L203
[ocamlviz]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/ocamlviz#L2241
[ocamlviz2]: https://github.com/ocaml/opam-bulk-logs/blob/2a43f506253b8e09004aa6499957b0b21658cf03/20140906/roo/4.02.0/raw/ocamlviz#L2279