release v1.15.1, prebuilt releases use stable Rust

ref #1053
ref shadowsocks/shadowsocks-android#2966

Author: zonyitoo

Cargo.lock

@@ -1,6 +1,6 @@
 [package]
 name = "shadowsocks-rust"
-version = "1.15.0"
+version = "1.15.1"
 authors = ["Shadowsocks Contributors"]
 description = "shadowsocks is a fast tunnel proxy that helps you bypass firewalls."
 repository = "https://github.com/shadowsocks/shadowsocks-rust"