Use `defadvice' rather than the new `nadvice` interface

Use `defadvice' rather than the new `nadvice` interface which is only available in trunk. This fixes f71b5f7.

* projectile.el (projectile-compilation-find-file): Convert to `defavice'.
                (projectile-mode): Use ad-(de)activate.


Note that this should only be a temporary fix. I'll try to patch `compile.el` to provide a hook so that we won't need this ugly advice.