.travis.yml: One stage for integration tests (userns and otherwise)

The separate stage is from 4964bd9 (test: run integration tests with
userNS enabled, 2018-05-24, #1578), but using a separate stage meant
that Travis serialized these slow (20+ minute) stages.  By flattening
into one stage we can run these in parallel (modulo Travis worker
availability) and get the Travis results earlier.

I've shifted the TEST_USERNS declaration into an env property so it
shows up in the Travis UI (e.g. [1]).

[1]: https://travis-ci.org/kubernetes-incubator/cri-o/builds/386122146

Signed-off-by: W. Trevor King <wking@tremily.us>

.travis.yml

@@ -59,9 +59,9 @@ jobs:
       script:
         - make integration
       go: "1.10.x"
-    - stage: Integration Test with User namespaces enabled
-      script:
-        - TEST_USERNS=1 make integration
+    - script:
+        - make integration
+      env: TEST_USERNS=1
       go: "1.10.x"
   allow_failures:
     - os: osx