Auto merge of #52776 - pietroalbini:bump-beta-bootstrap, r=Mark-Simulacrum

[beta] Bump bootstrap compiler to 1.27.2

This PR bumps the beta bootstrap compiler to 1.27.2.

* The original cargo version was wrong: the current one is not `0.28.0`, but `1.27.0` (since rust-lang/cargo#5083). [Should the forge be updated?](https://forge.rust-lang.org/release-process.html)
* The updated `date` is the correct one for rustc, but the wrong one for cargo. *I think* that shouldn't cause any problem, but it's better to ask.

r? @Mark-Simulacrum

Author: bors

src/stage0.txt

@@ -12,8 +12,8 @@
 # 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: 2018-06-21
-rustc: 1.27.0
+date: 2018-07-20
+rustc: 1.27.2
 cargo: 0.28.0
 
 # When making a stable release the process currently looks like: