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

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https://hg.mozilla.org/integration/gaia-central/rev/f542fff871d9
Author: Fernando Jiménez Moreno <ferjmoreno@gmail.com>
Desc: Merge pull request #15713 from jankeromnes/developer

Bug 963039 - [B2G][Settings] Developer settings are too hard to access. r=vingtetun

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https://hg.mozilla.org/integration/gaia-central/rev/c5ce933f7cd0
Author: Jan Keromnes <janx@linux.com>
Desc: Bug 963039 - [B2G][Settings] Developer settings are too hard to access r=vingtetun
Bumping manifests a=b2g-bump