vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/cloud.google.com/go/firestore/doc.go

@@ -21,6 +21,9 @@ database.
 See https://cloud.google.com/firestore/docs for an introduction
 to Cloud Firestore and additional help on using the Firestore API.
 
+Note: you can't use both Cloud Firestore and Cloud Datastore in the same
+project.
+
 Creating a Client
 
 To start working with this package, create a client with a project ID:
@@ -167,6 +170,7 @@ Call the Query's Documents method to get an iterator, and use it like
 the other Google Cloud Client iterators.
 
 	iter := q.Documents(ctx)
+	defer iter.Stop()
 	for {
 		doc, err := iter.Next()
 		if err == iterator.Done {