Bump Bazel version to 3.1.0

Change-Id: Ic52e49ad29e30cc9009dd5b313b6025b4b02cbe9
diff --git a/.bazelversion b/.bazelversion
index 4a36342..fd2a018 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.0.0
+3.1.0