Replace `e` alias with function to workaround Sierra issues.
Replace `e` alias with function to workaround Sierra issues.

`exa` version 0.4.0 is incompatible with macOS Sierra. Replace `e` alias
with a shell function that tests for this version's executible and falls
back to `ls` if 0.4.0 is the current `exa` version.

alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles,alphabetum/dotfiles