ci: travis: add coverity build job to be run from cron

ci: travis: add coverity build job to be run from cron

Summary:
We finally have regular Coverity Scan runs back to our CI.
It gets triggered from the cron jobs on Travis. As we are not able to
identify if it comes from a daily or weekly cron build I added a check
to only run the scan build on a Saturday so we should have a nice and
fresh report on Monday morning in our mailboxes.

Reviewers: zmike, bu5hm4n

Reviewed By: zmike

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9175