Update NDB to latest. (#1766)

There have been quite a few bug fixes in between.

Author: oliverchang

src/requirements.txt

@@ -11,7 +11,7 @@ google-cloud-core==1.3.0
 google-cloud-datastore==1.7.0
 google-cloud-logging==1.15.0
 google-cloud-monitoring==0.30.1
-google-cloud-ndb==1.0.0
+google-cloud-ndb==1.2.0
 google-cloud-profiler==1.0.3
 google-cloud-storage==1.13.2
 grpcio==1.28.1