servo: Merge #11040 - Add dbus to list of Debian build deps (from dagnir:dbus-build-dep); r=mbrubeck

Build failed without it on my Ubuntu 14.04 machine.  I can't confirm whether or not other OSes need it.

Source-Repo: https://github.com/servo/servo
Source-Revision: e52b9884d19b81dfc0e51986b1bbb89a227bfc4a

servo/README.md

@@ -41,7 +41,7 @@ sudo apt-get install git curl freeglut3-dev autoconf \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
     gperf g++ build-essential cmake virtualenv python-pip \
     libssl-dev libbz2-dev libosmesa6-dev libxmu6 libxmu-dev \
-    libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev
+    libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-dev libdbus-1-dev
 ```
 If you are on **Ubuntu 14.04** and encountered errors on installing these dependencies involving `libcheese`, see [#6158](https://github.com/servo/servo/issues/6158) for a workaround.