Corrected xrootd-client installation in slc6 test of .gitlab-ci.yml

.gitlab-ci.yml

@@ -6,9 +6,8 @@ test_navix_slc6_python35:
   when: manual
   image: mschnepf/slc6-condocker:test
   script:
-    # add python 3 and xrootd-client repositories
+    # add python 3 repositories
     - yum install -y sl-release-scl
-    - yum install -y epel-release
     # install python 3 and xrootd
     - yum install -y rh-python35 rh-python35-python-devel rh-python35-python-setuptools xrootd-client-devel
     # enable python 3