Auto merge of #52770 - pietroalbini:bump-bootstrap, r=Mark-Simulacrum

Bump bootstrap compiler

This PR bumps the bootstrap compiler to the latest beta, `beta-2017-07-27`. That should be the latest beta with relevant changes (the only nominated PR at the moment is a docs change).

r? @Mark-Simulacrum

Author: bors

src/stage0.txt

@@ -12,7 +12,7 @@
 # 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: 2018-07-13
+date: 2018-07-27
 rustc: beta
 cargo: beta