This no longer supports fish 2.2.0

[faho]

Since #28, this won't work on fish 2.2.0.

In particular it introduced a use of string (fish 2.3.0+), and it won't split $PATH and friends (fish 3.0+).

But the badge in the README still claims it supports fish >= 2.2.0.

Personally, I'd recommend just upping the requirements, especially since nobody complained in the 6 months since.

See also oh-my-fish/oh-my-fish#934.