Update .travis.yml

Author: lanwen

.travis.yml

@@ -4,6 +4,6 @@ jdk:
  - oraclejdk7
 
 after_success:
-  - mvn clean cobertura:cobertura org.eluder.coveralls:coveralls-maven-plugin:3.0.1:report
+  - mvn clean cobertura:cobertura -Dcobertura.report.format=xml org.eluder.coveralls:coveralls-maven-plugin:3.0.1:report
 notifications:
  email: false