add missing libyaml-dev dependency [for pyyaml]

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
diff --git a/Dockerfile b/Dockerfile
index 774220d..4d2b064 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,6 +15,7 @@
 RUN apt-get install -y geoip-database libgeoip1
 RUN apt-get install -y wget
 RUN apt-get install -y python-dev
+RUN apt-get install -y libyaml-dev
 
 RUN pip install django==1.7
 RUN pip install djangorestframework==2.4.4