Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/28a54cc75a9d
Author: Yuren Ju <yurenju@gmail.com>
Desc: Revert "Bug 971648: Add warning message if there is no manifest.webapp"

This reverts commit 3c7e468f78843992a892ff036f470ceb260c6a07.

========

https://hg.mozilla.org/integration/gaia-central/rev/980271752a87
Author: Yuren Ju <yurenju@gmail.com>
Desc: Revert "Bug 971648: Add warning message if there is no"

This reverts commit 70f3a23da3a89e00afda4eb3573de656b10d8ea0.
Bumping manifests a=b2g-bump