Bump bootstrap compiler to 1.28.0-beta.10

pietroalbini · pietroalbini · commit 04d31df71d4d · 2018-07-13T19:33:48.000+02:00
diff --git a/src/stage0.txt b/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
 
