Apply Coverity on the master branch

There are not much activity on the project so we shouldn't hit the
Coverity limits.

.travis.yml

@@ -15,4 +15,4 @@ addons:
     notification_email: nicolas+github@guillaumin.me
     build_command_prepend: "mvn clean"
     build_command:   "mvn -DskipTests=true compile"
-    branch_pattern: coverity
+    branch_pattern: master