Depend on broccoli 0.7.0 to avoid being detected as affected by the hardlink issue

The hardlink issue is being described here: https://github.com/joliss/broccoli/blob/master/docs/hardlink-issue.md

While `broccoli-requirejs` seems unaffected, the detection will suggest otherwise as this is depending on broccoli 0.3.0. Bump the version to at least 0.7.0