Add a Travis-CI job for systemd cgroup driver

The additional test shows as a separate job. It sets environment
RUNC_USE_SYSTEMD=1 so it will be clear in Travis-CI that this job is
testing the systemd cgroup driver.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

.travis.yml

@@ -7,6 +7,13 @@ go:
   - tip
 
 matrix:
+  include:
+    - go: 1.11.x
+      env:
+        - RUNC_USE_SYSTEMD=1
+      script:
+        - make BUILDTAGS="${BUILDTAGS}" all
+        - sudo PATH="$PATH" make localintegration RUNC_USE_SYSTEMD=1
   allow_failures:
     - go: tip