Check on APPROVED_APPLICATIONS instead of APPLICATIONS

APPROVED_APPLICATIONS can be a subset of APPLICATIONS when
some applications are set to supported=false on AMO
See https://bugzilla.mozilla.org/show_bug.cgi?id=1006654
Merge pull request #226 from yohanboniface/bug-1006654-check-approved-applications

Check on APPROVED_APPLICATIONS instead of APPLICATIONS