Fix broken Pillow pip dependency

The python Pillow dependency was not defined explicitly in askbot's
askbot_requirements.txt, so pip install always try to deploy the latest
version. Since 3.x Pillow also depends on libjpeg, so for a
successfull installation libjpeg-dev must be present.

Change-Id: Ia547034f128f46253ce857fed254531be71769b4

manifests/install.pp

@@ -15,6 +15,10 @@ class askbot::install (
     }
   }
 
+  package { 'libjpeg-dev':
+    ensure => present,
+  }
+
   if !defined(Package['virtualenv']) {
     package { 'virtualenv':
       ensure => present,