Merge branch 'stable-3.1'

* stable-3.1:
  Bump required Bazel version to 2.1.0

Change-Id: Ib93c351ff2024b5584a0b2ccb6e844ef479e03f5
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0