Upgrade rust/crates/crossbeam-epoch to 0.9.4

Test: make
Change-Id: Ib2bff9fb19c1fba16b132fa6e133b18683047ab6
diff --git a/README.md b/README.md
index c8ebd87..eb33bad 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@
 https://docs.rs/crossbeam-epoch)
 [![Rust 1.36+](https://img.shields.io/badge/rust-1.36+-lightgray.svg)](
 https://www.rust-lang.org)
-[![chat](https://img.shields.io/discord/569610676205781012.svg?logo=discord)](https://discord.gg/BBYwKq)
+[![chat](https://img.shields.io/discord/569610676205781012.svg?logo=discord)](https://discord.com/invite/JXYwgWZ)
 
 This crate provides epoch-based garbage collection for building concurrent data structures.