Bump Bazel version to 3.1.0

Change-Id: Ib6a0673fc8da2762d8af4b9948b5c762334d4973
diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0