Use Collection.stream instead of deprecated Collection.get in firestore

[petervandenabeele]

Fixing these deprecations (since 1.2.0 ?) when running pytest with google-cloud-firestore==1.2.0

snippets_test.py::test_quickstart_get_collection
  /Users/peter_v/data/projects/secretjobs/git/scraping/code/development/firestore/cloud-client/snippets.py:62: DeprecationWarning: 'Collection.get' is deprecated:  please use 'Collection.stream' instead.
    docs = users_ref.get()

[petervandenabeele]

Note: this is the API documentation for the deprecated get:
https://googleapis.github.io/google-cloud-python/latest/firestore/collection.html#google.cloud.firestore_v1.collection.CollectionReference.get

get(transaction=None)[source]
    Deprecated alias for stream().

[gguuss]

Thank you for the pull request, LGTM!