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

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https://hg.mozilla.org/integration/gaia-central/rev/8503ad943f74
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #17155 from jmcanterafonseca/fix_sim_names

Bug 926572 - [B2G][Contacts] Last name appears twice when merging an imp...

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https://hg.mozilla.org/integration/gaia-central/rev/7d39d480c66a
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 926572 - [B2G][Contacts] Last name appears twice when merging an imported SIM contact with Facebook contact
Bumping manifests a=b2g-bump