servo: Merge #9116 - Update virtualenv package name for Debian systems (from adrianheine:patch-1); r=jdm

Since Debian Jessie (current stable), the executable is in a `virtualenv` package.

Source-Repo: https://github.com/servo/servo
Source-Revision: b8fd8d9991a779854a66ed4099a19b9bceaf4435

servo/README.md

@@ -30,12 +30,14 @@ On Debian-based Linuxes:
 ``` sh
 sudo apt-get install curl freeglut3-dev \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
-    gperf g++ cmake python-virtualenv python-pip \
+    gperf g++ cmake virtualenv python-pip \
     libssl-dev libbz2-dev libosmesa6-dev libxmu6 libxmu-dev \
     libglu1-mesa-dev libgles2-mesa-dev libegl1-mesa-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.
 
+If `virtualenv` does not exist, try `python-virtualenv`.
+
 On Fedora:
 
 ``` sh