travis: enable testing with ceph "octopus"

Currently there's no container image named "octopus" but its imminent.
Once ceph container starts tagging images with "latest-octopus" we
should switch to that.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

.travis.yml

@@ -14,6 +14,9 @@ matrix:
     - env: CEPH_VERSION=luminous
     - env: CEPH_VERSION=mimic
     - env: CEPH_VERSION=nautilus
+    # use 'master' for soon-to-be-released 'octopus' until we have proper
+    # octopus named tags on containers
+    - env: CEPH_VERSION=master
 
 before_install: |
   make ci-image CEPH_VERSION=${CEPH_VERSION}