Check if ios directory exists before linking

[Ehesp]

I removed my ios directory, however when running the link command it fails with the error:

Installing Pods
[!] `/Users/.../ios` is not a valid directory.

Error occured during executing "node_modules/react-native-firestack/bin/cocoapods.sh" command
    at ChildProcess.prelink (/Users/.../node_modules/react-native/local-cli/core/makeCommand.js

Looks like it should be checking if it exists or not in the cocoapods file, but doesn't seem to work.

[auser]

Combining with #74