travis-ci: Adjust "branch_pattern"

Modify the "branch_pattern" for the Coverity add-on so that it also
matches "coverity/master" and "coverity/maint".

.travis.yml

@@ -48,4 +48,4 @@ addons:
     notification_email: team@monitoring-plugins.org
     build_command_prepend: tools/setup && ./configure
     build_command: make
-    branch_pattern: coverity
+    branch_pattern: coverity.*