Fix/configure sender for email notification

Hi, after moving from to notifications have stopped working for the apache/httpd project.

The email notifications used to use an enveloper sender of “” but now use an (unpredictable?) Amazon SES address like - this is hard/impossible to whitelist safely when we are delivering to a mailing list.

Could either

a) the enveloper sender be changed to a fixed address, or
b) allow us to configure the enveloper sender in .travis.yml?

TIA, Joe Orton