Bump Bazel version to 3.1.0

Change-Id: I3faf0c851a68ac6bed82b214b207735673172154
diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0