Exlde gstreamer when building XPIs
Allow overide on MOZ_APP_MAXVERSION

Adds option --soft-max-version.  This allows a 'soft' version number thus an
Aurora XPI of 24.0a2 will have an maximum compatible version of 24.0.* instead
of 24.0a2. This should allow someone to download an Aurora language pack and
use it in the final release.

Fix bug 3047