Setup Danger to prevent Google Play services being included again

Quote from @hboon

FYI. We have a Danger file at https://github.com/AlphaWallet/alpha-wallet-ios/blob/master/dangerfile.js that contains code to check for such things for iOS. It's just Javascript with convenient access to the code changes. So we have simple checks like if a string is added to the en localisation, we at least warn that other localisations should be added for other messages.

It can be configured so it's run automatically with Travis for PRs. We haven't done that yet, so I'm just running it as a post-commit hook locally:

#!/bin/sh
yarn run danger local