Build cobertura report only on master for successful builds

Building the report takes time, and when someone submits a pull request,
we should not show it as failure if the build timeouts for the report.

We only need the report when we build master, and only if the build is
successful.

Author: Franck Cuny <fcuny@apache.org>

Reviewers: Sijie Guo <sijie@apache.org>

Closes #45 from franckcuny/fcuny/fix-travis

.travis.yml

@@ -24,9 +24,12 @@ before_install:
   - echo "MAVEN_OPTS='-Xmx3072m -XX:MaxPermSize=512m'" > ~/.mavenrc
 
 script:
-  - travis_retry mvn clean apache-rat:check
-  - travis_retry mvn clean install findbugs:check -DskipTests=true
-  - travis_wait 60 mvn package cobertura:cobertura coveralls:report
+  - travis_retry mvn --batch-mode clean apache-rat:check
+  - travis_retry mvn --batch-mode clean install findbugs:check -DskipTests=true
+
+# we build the report only after a successful build on master
+after_success:
+  - $TRAVIS_BRANCH == 'master' && travis_wait 60 mvn --batch-mode clean cobertura:cobertura coveralls:report
 
 cache:
   directories: