Builder should run createrepo

Testing Done:
Tested on 0.9.x branch

Reviewed at https://reviews.apache.org/r/38084/
diff --git a/builder/rpm/centos-7/build.sh b/builder/rpm/centos-7/build.sh
index 912f7c7..be3c6fb 100755
--- a/builder/rpm/centos-7/build.sh
+++ b/builder/rpm/centos-7/build.sh
@@ -29,3 +29,7 @@
 make srpm
 yum-builddep -y ../../../dist/rpmbuild/SRPMS/*
 make rpm
+
+yum -y install createrepo
+cd ../../../dist/rpmbuild/RPMS/x86_64
+createrepo .