travis: use "octopus" release by name

Within the last 24hrs the "octopus" release of ceph is available, and
matching tags have been created in the ceph containers project. Consume
the release name rather than master.

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

.travis.yml

@@ -14,9 +14,7 @@ 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
+    - env: CEPH_VERSION=octopus
 
 before_install: |
   make ci-image CEPH_VERSION=${CEPH_VERSION}