Bump required Bazel version to 2.1.0

Change-Id: I9060941177852b9cb9c26043dfc06681f12d89af

.bazelversion

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