bump spin to v0.4.6 (instead of a git ref)

Cargo.toml

@@ -9,15 +9,12 @@ crate-type = ["staticlib"]
 [dependencies]
 rlibc = "1.0"
 volatile = "0.2"
+spin = "0.4.6"
 x86_64 = "0.1.2"
 multiboot2 = "0.3.2"
 bitflags = "0.9"
 linked_list_allocator = "0.4.2"
 
-[dependencies.spin]
-git = "https://github.com/messense/spin-rs"
-rev = "020f1b3"
-
 [dependencies.log]
 version = "0.3"
 default-features = false