Bump required Bazel version to 2.1.0

Change-Id: Ia5191ffc2c236db322f7e1ccfa7db175d8246af0
diff --git a/.bazelversion b/.bazelversion
index 227cea2..7ec1d6d 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-2.0.0
+2.1.0