Rollup merge of rust-lang#52350 - pietroalbini:bump-bootstrap, r=Mark-Simulacrum

Bump bootstrap compiler to 1.28.0-beta.10

r? @Mark-Simulacrum

Author: kennytm

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: 2018-06-30
+date: 2018-07-13
 rustc: beta
 cargo: beta