Bump required Bazel version to 2.1.0

Change-Id: I567a29f4790d7bd53a5ba3ea6535018fa0c3bcd0
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0