Bazel: add .bazelversion.

Change-Id: I56448c565c6f8e343138a03b4f8f0f37ff25adb8
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://nginx-review.googlesource.com/c/nginx/+/3648
Reviewed-by: Daniel Berkovitch <dbrk@google.com>
diff --git a/.bazelversion b/.bazelversion
new file mode 100644
index 0000000..47b322c
--- /dev/null
+++ b/.bazelversion
@@ -0,0 +1 @@
+3.4.1