Require explicit conversion from allocators.

(Not what it says in the draft standard, but I think that might be a defect).


[SVN r47085]

libs/unordered

@@ -1 +1 @@
-Subproject commit 3640e1e96ca4cfa8feed9132e3cb89553fa77092
+Subproject commit d66936a64000ec5b21e79ce79bad5ed7b48cb683