Update bootstrap compiler

Author: alexcrichton

src/stage0.txt

@@ -12,9 +12,9 @@
 # source tarball for a stable release you'll likely see `1.x.0` for rustc and
 # `0.x.0` for Cargo where they were released on `date`.
 
-date: 2017-07-18
-rustc: beta
-cargo: beta
+date: 2017-08-27
+rustc: 1.20.0
+cargo: 0.21.0
 
 # When making a stable release the process currently looks like:
 #
@@ -34,4 +34,4 @@ cargo: beta
 # looking at a beta source tarball and it's uncommented we'll shortly comment it
 # out.
 
-#dev: 1
+dev: 1