Bumping gaia.json for 4 gaia-central revision(s) a=gaia-bump

========

https://hg.mozilla.org/integration/gaia-central/rev/97819e905b60
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Merge pull request #15151 from jmcanterafonseca/bug_add_more_unit_tests_fb

Bug 957529 - Add more unit tests for fb_data modules (error conditions, ...

========

https://hg.mozilla.org/integration/gaia-central/rev/d065c8da3d1c
Author: Jose M. Cantera <jmcanterafonseca@gmail.com>
Desc: Bug 957529 - Add more unit tests for fb_data modules (error conditions, particularly datastore)

========

https://hg.mozilla.org/integration/gaia-central/rev/8f80a9f7662c
Author: George <georgeiscoming@gmail.com>
Desc: Merge pull request #14837 from cctuan/869289

Bug 869289 - Video app fails to list videos where it can't generate a th...

========

https://hg.mozilla.org/integration/gaia-central/rev/528d6401e72f
Author: cctuan <georgeiscoming@gmail.com>
Desc: Bug 869289 - Video app fails to list videos where it can't generate a thumbnail