travis: change coverity to run on master

This change goes with disabling:
- Build pushed branches
- Build pushed pull requests

And adding a cron job on travis-ci to build master weekly.

.travis.yml

@@ -9,7 +9,9 @@ matrix:
   include:
     - os: linux
       dist: xenial
-      if: branch = coverity_scan
+      # In order to stay under the coverity rate limit, we only run this weekly
+      # and not on push which is configured in travis-ci settings.
+      if: branch = master
 
 before_install:
     - echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca-