travis: Disable most suites in favor of CircleCI!

Circle has been much more robust than Travis in the few weeks we've
had them both enabled, as well as somewhat faster.  By disabling
Travis, we're no longer exposed to its spurious failures.

One suite remains on Travis: the "production" suite.  This work is
being tracked in #7748.

.travis.yml

@@ -48,14 +48,9 @@ matrix:
     # that doesn't seem to be documented, but it's what we see empirically.
     # We only get 4 jobs running at a time, so we try to make the first few
     # the most likely to break.
-    - python: "3.4"
-      env: TEST_SUITE=frontend
-    - python: "3.4"
-      env: TEST_SUITE=backend
     - python: "3.4"
       env: TEST_SUITE=production
-    - python: "3.5"
-      env: TEST_SUITE=backend
+    # Other suites moved to CircleCI -- see .circleci/.
 sudo: required
 addons:
   artifacts: