Bump required Bazel version to 2.1.0

Change-Id: Ia0e8890b5cce9d953daa133ef4c5aba5c06e2a2f
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0