Include build script on Git to allow installation of in-dev releases

[Mark-H]

Include build on Git to allow installation of in-dev releases.

Main reason I'm requesting this now is to get it working on 2.2, without having to manually apply the fixes.

[Bruno17]

I think it doesn't make much sense to install the mediasoures-branch of MIGX, because the mediasources-branch is actually only working correctly with mods to revo I made on this pull-request:
modxcms/revolution#238

perhaps you want to vote for this pull-request. Would be a great improvement to mediasources to have them more dynamic.