travis: add race detector job for Swarm

We had more than a dozen of Data Races in our code base. Go has
a nice builtin race detector tool. So we should have a CI job
that at least notifies us timely if we introduce a new race.

1st let's just have a daily CRON job triggered on
ethersphere/go-ethereum. Later as the job gets stable, we can start
using it for ethersphere PRs.

resolves: ethersphere/go-ethereum#741

.travis.yml

@@ -214,3 +214,12 @@ matrix:
         submodules: false # avoid cloning ethereum/tests
       script:
         - go run build/ci.go purge -store gethstore/builds -days 14
+
+    - name: Race Detector for Swarm
+      if: repo = ethersphere/go-ethereum AND type = cron
+      os: linux
+      dist: trusty
+      go: 1.11.x
+      git:
+        submodules: false # avoid cloning ethereum/tests
+      script: ./build/travis_keepalive.sh go test -v -timeout 20m -race ./swarm...