travis-ci: Don't run coverity_scan on master branch

Once the weekly limit for Coverity scan is reached all builds exit
success without compiling.

.travis.yml

@@ -34,4 +34,4 @@ addons:
     notification_email: quake3-commits@icculus.org
     build_command_prepend: "make clean"
     build_command: "make release"
-    branch_pattern: master
+    branch_pattern: coverity_scan