Support for `CORE_RELEASE=3@stable`

[chillu]

This will help to reduce complexity in module build matrices. We're speeding up the frequency of minor releases (less than a month between 3.3 and 3.4), so its becoming infeasible for module maintainers to keep on top of all the build variations in their travis.yml.

• Add support in parseComposerConstraint()
• Replace git clone in travis-setup.php with composer install silverstripe/installer $CORE_RELEASE --no-install
• Fix version_compare() in travis-setup.php
• Fix version_compare() in _ss_environment.php

[tractorcow]

Related: silverstripe/silverstripe-framework#6336