Implement the SOURCE_DATE_EPOCH specification

[sathieu]

See https://reproducible-builds.org/specs/source-date-epoch/

This is necessary to have reproducible builds.

Bug-Debian: https://bugs.debian.org/750697

[sathieu]

The build is failing because of https://travis-ci.community/t/php-5-4-and-5-5-archives-missing/3723.

Do you want to drop support for php < 5.6 or force the use of trusty for those old versions?

[sathieu]

See also https://changelog.travis-ci.com/xenial-as-the-default-build-environment-99476.

[kenguest]

Thanks for this!

[sathieu]

Thanks for merging!

[kenguest]

You did the work though! Also, I never heard of the SOURCE_DATE_EPOCH specification before but it seems very interesting and useful.

In relation to dropping support for php < 5.6, I think we need to force the use of trusty for those older versions, (5.4 and 5.5) as they do appear to still be in use judging by internal logs.