mk: Bootstrap from the 1.9.0 stable release

Author: alexcrichton

src/stage0.txt

@@ -12,6 +12,6 @@
 # tarball for a stable release you'll likely see `1.x.0-$date` where `1.x.0` was
 # released on `$date`
 
-rustc: beta-2016-04-13
-rustc_key: c2743eb4
+rustc: 1.9.0-2016-05-24
+rustc_key: d16b8f0e
 cargo: nightly-2016-04-10