Add .env support in parallel to _ss_environment.php

[chillu]

4.0 no longer supports _ss_environment.php, so we need some conditional logic on how to initialise projects, based on them running 3.x or 4.x. See silverstripe/silverstripe-framework#6337

https://github.com/silverstripe/silverstripe-travis-support/blob/master/_ss_environment.php