Add coveralls parallel builds webhook (#8374)

Since we use parallel builds for unit tests, follow the instructions for
properly merging the results on coveralls:
https://docs.coveralls.io/parallel-build-webhook

.travis.yml

@@ -27,6 +27,7 @@ cache:
 
 env:
   global:
+    - COVERALLS_PARALLEL=true
     - DOCKER_IP=127.0.0.1  # for integration tests
     - MVN="mvn -B"
     - > # Various options to make execution of maven goals faster (e.g., mvn install)
@@ -42,6 +43,9 @@ env:
       "
     - MAVEN_SKIP_TESTS="-DskipTests -Djacoco.skip=true"
 
+notifications:
+  webhooks: https://coveralls.io/webhook
+
 # Add various options to make 'mvn install' fast and skip javascript compile (-Ddruid.console.skip=true) since it is not
 # needed. Depending on network speeds, "mvn -q install" may take longer than the default 10 minute timeout to print any
 # output.  To compensate, use travis_wait to extend the timeout.