getCollections declaration is missing in firestore.d.ts

[MarkChrisLevy]

Environment details

• OS: MacOs 10.13
• Node.js version: 8.1.4
• npm version: 5.5.1
• @google-cloud/firestore version: 0.8.2

Steps to reproduce

Typescript declaration file (firestore.d.ts) is missing getCollections() method for Firestore class and DocumentReference class.

[schmidt-sebastian]

Thanks for pointing this out! PR is coming.