Cleaned up activate.fish.

Cleaned up __activate.fish__ to be more *idiomatic* fish and removed a bit of unnecessary code (such as support for "Aspen magic directories", since *AFAIK* and *[AFAICT](http://aspen.io/virtual-paths 'Aspen: Python Web Framework with Simplates')*, this is no longer the case).

\- Jonathan
We shouldn't add alternatives for spelling here selectively, especially since its inconsistent with A) other specified public env vars (e.g `$VIRTUAL_ENV`), and B) the names of those vars for other shells as well.

If you'd like to suggest to rename them, I'd suggest this should be done in a separate PR covering the other activate scripts as well so it would be consistent across the board.