Do not `expand()` shell vars

This is especially important when testing/using variables that do not
exist, because then a new shell is spawned by Vim.
But since they should beavailable already in Vim from the (shell) env
there is no point in doing so.

See also: http://www.reddit.com/r/vim/comments/2el2zo

It is not really relevant here, because `$SHELL` will typically exists, but a good habit anyway.