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

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https://hg.mozilla.org/integration/gaia-central/rev/300c3198dcd3
Author: Zac <zcampbell@mozilla.com>
Desc: Merge remote-tracking branch 'davehunt/bug_984340'

Conflicts:
	tests/python/gaia-ui-tests/gaiatest/tests/tbpl-manifest.ini

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https://hg.mozilla.org/integration/gaia-central/rev/75748cb1ce64
Author: Dave Hunt <dave.hunt@gmail.com>
Desc: Bug 984340 - Enable sdcard tests on desktop B2G. r=bsilverberg
Bumping manifests a=b2g-bump