Bump Bazel version to 3.1.0

Change-Id: I50b2bef926c506c20527b6293608c0b2837e4dd2

.bazelversion

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