Bump required Bazel version to 2.1.0

Change-Id: I0176680524a5a1959cb2e307b8f7015839c793a7

.bazelversion

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