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

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https://hg.mozilla.org/integration/gaia-central/rev/c48e91abf8e7
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Merge pull request #15744 from mikehenrty/bug-959800-mozbrowser

Bug 959800 - [Offline] mozbrowser iframes don't display correct offline ...

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https://hg.mozilla.org/integration/gaia-central/rev/45d0079782f5
Author: Michael Henretty <michael.henretty@gmail.com>
Desc: Bug 959800 - [Offline] mozbrowser iframes don't display correct offline content
Bumping manifests a=b2g-bump