Added libbz2-dev to Debian based prerequisites

Encountered this when building for the first time today:

```
note: /usr/bin/ld: cannot find -lbz2
collect2: error: ld returned 1 exit status

error: aborting due to previous error
Could not compile `servo`.
```

Installing the libbz2-dev packaged fixed it.

README.md

@@ -28,7 +28,7 @@ On Debian-based Linuxes:
 sudo apt-get install curl freeglut3-dev \
     libfreetype6-dev libgl1-mesa-dri libglib2.0-dev xorg-dev \
     msttcorefonts gperf g++ cmake python-virtualenv \
-    libssl-dev
+    libssl-dev libbz2-dev
 ```
 
 On Fedora: