CI: Install gems as unprivileged user

Installing gems as user root and accessing them as
unprivileged user breaks with gems that ship files
with too restrictive file permissions.

See https://github.com/invadersmustdie/puppet-catalog-test/issues/38

.gitlab-ci.yml

@@ -9,7 +9,7 @@ cache:
 
 before_script:
   - cd tests/platform-ci
-  - time ./setup.sh
+  - su -c 'time ./setup.sh' cirunner
 
 stages:
   - syntax