Bumping gaia.json for 3 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/d983f74965c0
Author: Hubert Figuière <hub@figuiere.net>
Desc: Merge pull request #15172 from hfiguiere/bug917717

Bug 917717 - Collect the memory usage for the process after running the start test. r=vivien

========

https://hg.mozilla.org/integration/gaia-central/rev/20adabead945
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 917717 - Part 2: nits.

========

https://hg.mozilla.org/integration/gaia-central/rev/c95347535db6
Author: Hubert Figuière <hub@figuiere.net>
Desc: Bug 917717 - Collect the memory usage for the process after running the start test.