Bump Bazel version to 3.1.0

Change-Id: I8a6e9fb4047b3a4c167b37a787feebe6f9bfb6f3
diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0