Fix dependencies

What does this MR do?

• php needs httpd so we can run brew test php
• python3 needs to exist also on old OSes where python2 is the default
• fastlane on 10.13 is a silent failure: Running: gem install fastlane --version 2.63.0 ... FAIL: ERROR: Error installing fastlane: signet requires Ruby version >= 2.5.
• cocoapods needs setup on mojave and high sierra
• homebrew needs to stop autoupdating everytime we run the first brew command
• high sierra needs a more recent brew checkout so brew test works with our version of the command line tools. It also updates the location of the caskroom and PHP becomes available
• flutter needs to download a bunch of dependencies

Why was this MR needed?

Working on tests in !90 (merged)

What's the best way to test this MR?

What are the relevant issue numbers?