Allow to run apollo codegen:generate --watch on non tty devices

[trojanowski]

Currently it's not possible to use the --watch options if the stdin is not a tty (e. g. if the command is run using the concurrently cli tool). You can reproduce this bug with the following command:

apollo codegen:generate --target=typescript --schema=src/data/schema.json '--queries=src/**/*.tsx' --watch < /dev/null

You'll see the following output:

 ✔ Loading Apollo config
 ✔ Scanning for GraphQL queries (6 found)
 ✔ Generating query files with 'typescript' target - wrote 3 files
Press any key to stop.
TypeError: process.stdin.setRawMode is not a function
    at waitForKey (~/Projects/omnicalc/node_modules/apollo/lib/commands/codegen/generate.js:17:19)
    at Generate.run (~/Projects/omnicalc/node_modules/apollo/lib/commands/codegen/generate.js:127:23)
    at <anonymous>

This PR fixes the problem.

[apollo-cla]

@trojanowski: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[shadaj]

Thanks for the fix!