Bump Bazel version to 3.1.0

Change-Id: Icd4dd296c156183545d343a21f45cd30a736d51b
diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0