(maint) - travis_el6 fix

Issues with the current images so swapping to use different ones

Author: David Swan

provision.yaml

@@ -1,6 +1,6 @@
 ---
 default:
-  provisioner: docker_exp
+  provisioner: docker
   images:
   - litmusimage/centos:7
 vagrant:
@@ -9,7 +9,7 @@ vagrant:
   - centos/7
   - generic/ubuntu1804
 travis_deb:
-  provisioner: docker_exp
+  provisioner: docker
   images:
   - litmusimage/debian:8
   - litmusimage/debian:9
@@ -28,12 +28,12 @@ travis_ub_6:
   - litmusimage/ubuntu:18.04
   - litmusimage/ubuntu:20.04
 travis_el6:
-  provisioner: docker_exp
+  provisioner: docker
   images:
-  - litmusimage/centos:6
-  - litmusimage/scientificlinux:6
+  - centos:6
+  - scientificlinux/sl:6
 travis_el7:
-  provisioner: docker_exp
+  provisioner: docker
   images:
   - litmusimage/centos:7
   - litmusimage/oraclelinux:7