Remove references to Travis

Summary:
react-native project is migrated travis to circleCI.
but the document has some leavings.

Only Docs Edited

https://github.com/facebook/react-native/pull/16354
https://github.com/facebook/react-native/pull/16364

[ DOCS] Remove references to Travis
changed Releases.md
changed RNTester/RNTesterIntegrationTests/RNTesterSnapshotTests.m
changed scripts/android-e2e-test.js
Closes https://github.com/facebook/react-native/pull/17733

Differential Revision: D6806336

Pulled By: shergin

fbshipit-source-id: 22d21995d718a89bb831f75ed2ff6074dd22c6fe