Upgrade base Python version (#330)

diff --git a/Dockerfile b/Dockerfile
index b72aa2c..ca172bb 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM python:2.7.15
+FROM python:2.7.16
 
 # install librdkafka
 ENV LIBRDKAFKA_VERSION 0.11.6