updates to smarter dependency requirements (based on semantic versioning)

This commit updates composer.json to require dependencies based on what semantic versioning says should work. As you can [read here](http://semver.org/) a minor version change should only implement backwards-compatible changes. As such, we can assume that anything from Laravel >= 4.0, but < 5 is safe to use. Additionally, anything from stripe/stripe-php > 1.9, but < 2 should be safe to use.

This update should make updates to the Stripe API and Laravel much easier to upgrade to.