Permit Hermes compilation without launching XCode.

The associated error:
$ make hermes
... SNIP ...
xcodebuild: error: The project 'Hermes' does not contain a scheme named 'Hermes'.

This issue is documented on the Travis CI website & StackOverflow.
- http://docs.travis-ci.com/user/languages/objective-c/#Default-Test-Script
- http://stackoverflow.com/a/16049998/2720026

Marking schemes as shared also allows Hermes compilation on CI services.