Bump required Bazel version to 2.1.0

Change-Id: Ibbd141f8b1b428501d224aa5637a0e1e90ef3cde
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0