Change plugin to source the latest version of fisher from the upstream repository jorgebucaran/fisher

[frederickjh]

The current version of this plugin is using static files in its repository to install fisher. It would be better to download these files directly from the upstream repository jorgebucaran/fisher to get the latest version. Version 4.0 is a complete rewrite.

[frederickjh]

Upstream is giving the following instructions to install:

curl -sL https://git.io/fisher | source && fisher install jorgebucaran/fisher

Basically fisher is installing itself using a temporary copy of itself downloaded to stdin. Need to see if this will also work with $XDG_CONFIG_HOME being set to $HOME/.xxh/.config.