Upgrade CentOS to v7.5.1804

Change-Id: I7981700cbb212eb0c13ce5c708ae5a8323469567
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index e4ef1c5..85d9cae 100644
--- a/centos/7/Dockerfile
+++ b/centos/7/Dockerfile
@@ -1,4 +1,4 @@
-FROM centos:7.4.1708
+FROM centos:7.5.1804
 MAINTAINER Gerrit Code Review Community
 
 RUN yum -y install initscripts sudo