Summary:
I cherry-picked this PR which:

1. rename everything to `hermes-engine` since <https://cocoapods.org/pods/Hermes> already exists
2. conditionalize `spec.source` so it can distribute prebuilt binaries as alloy suggested at https://github.com/facebook/hermes/issues/368#issuecomment-702041567

Noted that the version in the `podspec` had been updated to 0.7.1
correctly.

Pull Request resolved: https://github.com/facebook/hermes/pull/385

Test Plan: CircleCI doing fine

Reviewed By: tmikov

Differential Revision: D24206180

Pulled By: Huxpro

fbshipit-source-id: ec46350e00099c61a7de8cc9eb99991af72abdb4