pjproject_bundled: Use the asterisk github mirror for download

We now mirror the pjproject tarball and md5 at
https://github.com/asterisk/third-party/tree/master/pjproject

To improve download reliability, we now get the tarball from
our mirror instead of from pjsip.org.

ASTERISK-27052 #close
Reported-by: 'alex'

Change-Id: I60236587a8935bfa71fcc391f4e2ecb31918c08a