"settings" lacks a return type annotation

[laurenzlong]

Thanks for stopping by to let us know something could be better!

Please run down the following list and make sure you've tried the usual "quick
fixes":

• Search the issues already opened: https://github.com/googleapis/nodejs-firestore/issues
• Search the issues on our "catch-all" repository: https://github.com/GoogleCloudPlatform/google-cloud-node
• Search StackOverflow: http://stackoverflow.com/questions/tagged/google-cloud-platform+node.js

If you are still having issues, please be sure to include as much information as
possible:

Environment details

• OS: OS X
• Node.js version: 6.11.5
• npm version: 3.10.10
• @google-cloud/firestore version: 0.15.2

Steps to reproduce

firestore.settings({ timestampsInSnapshots: true })

Running "tsc" produces the error:

node_modules/@google-cloud/firestore/types/firestore.d.ts(105,5): error TS7010: 'settings', which lacks return-type annotation, implicitly has an 'any' return type

I was able to make this go away by changing my tsconfig.json to have "noImplicityAny: false", but it would be great if I didn't have to do this.