Merge branch 'stable-3.1'

* stable-3.1:
  Bump required Bazel version to 2.1.0

Change-Id: I18ecc81953c6084a8828d8a29d93dd9be19c5fa7

.bazelversion

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