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

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https://hg.mozilla.org/integration/gaia-central/rev/6688b19ae7fa
Author: gitmai <mri@tid.es>
Desc: Merge pull request #15644 from gitmai/bug-960500-CC-specific-message-for-airplanemode

Bug 960500 - [Buri][Data usage] Show a specific message when launching d...
r=salva

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https://hg.mozilla.org/integration/gaia-central/rev/7f71d3d6d849
Author: mai <mri@tid.es>
Desc: Bug 960500 - [Buri][Data usage] Show a specific message when launching data usage app under airplane mode scenario
Bumping manifests a=b2g-bump