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

========

https://hg.mozilla.org/integration/gaia-central/rev/7dc42c64c26c
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Merge pull request #16269 from stasm/972440-memory-average

Bug 972440 - Provide average memory measurements. r=hfiguiere

========

https://hg.mozilla.org/integration/gaia-central/rev/8cd936c399ae
Author: Staś Małolepszy <stas@mozilla.com>
Desc: Bug 972440 - Provide average memory measurements. r=hfiguiere
Bumping manifests a=b2g-bump