update powershell recipe

Recreated from #1308.

I addressed @purcell's feedback for [powershell.el](https://github.com/jschaf/powershell.el) with the following changes.  Also, thanks for the review.  Let me know if needs any other tweaks.

* Emacs 24 is listed in package-requires
* Used unconditional loading for compile and soft-loading for speedbar.