Add zlib dev library to pre-req packages list.

GettingStarted.wiki

diff --git a/GettingStarted.wiki b/GettingStarted.wiki
index 7ff72ce..7de85c3 100644
--- a/GettingStarted.wiki
+++ b/GettingStarted.wiki
@@ -24,6 +24,7 @@
  * python-lxml
  * virtualenv (version 1.10 or later)
  * xvfb
+ * zlib1g-dev
 
 == Setting up ==