Merge branch 'stable-3.1'

* stable-3.1:
  Bump required Bazel version to 2.1.0

Change-Id: If99cba18214c449a76d0ce09e493bb8dc3694d38
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0