update google-auth to 1.15.0 final part [(#3819)](GoogleCloudPlatform/python-docs-samples#3819)

Author: leahecole

samples/snippets/api/requirements.txt

@@ -1,3 +1,3 @@
 google-api-python-client==1.8.2
-google-auth==1.14.3
+google-auth==1.15.0
 google-auth-httplib2==0.0.3