Improve composer consistency
Remove minimum-stability from composer.json
Fix version discrepency

CoreBundle requires knplabs/gaufrette in version 0.2.*@dev, Core
component should require the same version instead of *@dev.
Require dev version of rest-bundle
Fix typo in doctrine/collections package name
Add missing dependencies
Update composer.json

Dependencies from sub bundles have been added to composer.json as
composer doesn't recurse into replaced libraries.
Omnipay is required as long as it doesn't appear in payum/payum-bundle
composer.