Merge branch 'stable-3.1'

* stable-3.1:
  Bump required Bazel version to 2.1.0

Change-Id: Ia33b467b82ef58f30b34a20ad2c8ff6b7df144d4
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0