Bump CentOS to 7.4.1708

Change-Id: Ia03f0ebb69f6a319dcbfb7c0e88250d18f4381fe
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index 277ae9c..221c341 100644
--- a/centos/7/Dockerfile
+++ b/centos/7/Dockerfile
@@ -1,4 +1,4 @@
-FROM centos:7.3.1611
+FROM centos:7.4.1708
 MAINTAINER Gerrit Code Review Community
 
 # Allow remote connectivity and sudo