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

========

https://hg.mozilla.org/integration/gaia-central/rev/78785ff91c6d
Author: Yuren Ju <yurenju@gmail.com>
Desc: Merge pull request #21152 from yurenju/add-build-jshint

Bug 1031992 - enable jshint for build direcotry r=@cctuan

========

https://hg.mozilla.org/integration/gaia-central/rev/da939e8fa98b
Author: Yuren Ju <yurenju@gmail.com>
Desc: Bug 1031992 - enable jshint for build direcotry
Bumping manifests a=b2g-bump