Update the CHANGELOG for v2.5.4 of Oppia#3879

Merged

BenHenning merged 2 commits intodevelopfrom update-changelog

Sep 22, 2017

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Commits on Sep 21, 2017

Update the CHANGELOG for v2.5.4 of Oppia.
BenHenning
committed

Commits on Sep 22, 2017

Update changelog to refer to the correct date.
BenHenning
committed