Update bootstrap compiler from 1.31.0-beta.1 to 1.31.0-beta.14

This includes rust-lang#55474 which is needed to build rust-lang#56066

Author: Jethro Beekman

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: 2018-10-30
+date: 2018-11-19
 rustc: beta
 cargo: beta