Summary:
Adding shebang to local-cli's setup_env script.
Depending how your *nix environment is setup, running `react-native bundle` will throw this error:
```
Error: spawnSync /opt/build/in/node_modules/react-native/local-cli/setup_env.sh Unknown system error -8
at exports._errnoException (util.js:1022:11)
at spawnSync (child_process.js:461:20)
at Object.execFileSync (child_process.js:498:13)
at repl:1:14
at sigintHandlersWrap (vm.js:22:35)
at sigintHandlersWrap (vm.js:96:12)
at ContextifyScript.Script.runInThisContext (vm.js:21:12)
at REPLServer.defaultEval (repl.js:346:29)
at bound (domain.js:280:14)
at REPLServer.runBound [as eval] (domain.js:293:12)
```
In my case I was running react-native on docker (using alpine-node:6 as base image).
Closes https://github.com/facebook/react-native/pull/12173
Differential Revision: D5154069
Pulled By: shergin
fbshipit-source-id: acf2a21499b4a57310afd06b57386e7900662b7d
f16df60b8d