Upgrade CentOS to 7.5.1804

Change-Id: Ic3d7465410da30747ef11555f05c2c64b2bdbdcc
diff --git a/centos/7/Dockerfile b/centos/7/Dockerfile
index b02a62b..db2d1e2 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