Bump required Bazel version to 2.1.0

Change-Id: I277fe449de56c8c0435b1b0f1368ad5a91b1fe36

.bazelversion

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