emacs.pkgs.ada-mode: fix installPhase

The invocation of the install.sh script changed in 8.1.0 or 8.0 to no
longer accept the --prefix flag, instead the path needs to be given as
the first argument alone.

pkgs/applications/editors/emacs/elisp-packages/elpa-packages.nix

@@ -98,7 +98,7 @@ self: let
         '';
 
         postInstall = (old.postInstall or "") + "\n" + ''
-          ./install.sh --prefix=$out
+          ./install.sh "$out"
         '';
 
         meta = old.meta // {