Bump bazel version to 3.4.0

The new Bazel release includes major Error Prone upgrade to 2.4.0.

Change-Id: I41928d63fade6e9bcf4beafd48c2bcc57988d410

.bazelversion

diff --git a/.bazelversion b/.bazelversion
index fd2a018..1809198 100644
--- a/.bazelversion
+++ b/.bazelversion
@@ -1 +1 @@
-3.1.0
+3.4.0