Fix 'Unexpected strict mode reserved word', bump version to 1.3

Summary:
Looks like using 'let' was too ambitious :)

This should fix the issue https://github.com/facebook/react-native/issues/10976.

**Test plan (required)**

Travis & Circle CI tests on this PR.
Closes https://github.com/facebook/react-native/pull/11166

Differential Revision: D4237047

Pulled By: bestander

fbshipit-source-id: 85b548d74a0847621519364201cf71d6649ad605