Automatically enable PHP Mode for Amaka scripts

See the project page

    http://trashofmasters.github.io/amaka/

for more information about the Amaka build tool.  Amaka’s build
scripts are PHP files, and so this patch adds `Amkfile` (the name of
those scripts) to `auto-mode-alist` so that Emacs will enable PHP Mode
when viewing or editing them.