Simplified travis builds (external setup scripts)
composer/installers dep workaround

Moved to end of requirements, to work around a bug in composer - see https://github.com/composer/composer/issues/1147. This caused the dependencies to be installed in the wrong folder because the custom 'silverstripe-module' instructions hadn't been loaded at the time the core modules were installed via composer.
Merge remote-tracking branch 'origin/2.4' into 3.0

Conflicts:
	.travis.yml
	composer.json