Preserve fish as 'shell' for Vim 7.4.276 or later

[jamessan]

As of 7.4.276, Vim understands how to run commands in subshells when
'shell' is fish. This fixes the most common problems with running
external commands from Vim.

Plugins may still need to accommodate for fish, but fixing the cognitive
dissonance of having the user's shell changed outweighs the benefit to
lazy plugin writers.

[jsatk]

I believe this commit causes some issues with tools like Dispatch. See here: tpope/vim-dispatch#243 (comment).

@jamessan are you able to get Dispatch to work in Vim 8 with Fish shell?

[tpope]

Other plugins, even ones I wrote, are not Sensible's problem.