improv: set shell to ZSH in interactive mode

In interactive mode, ZSH will read `~/.zshrc`. Else, it will first stick to
`/etc/zshenv`, then `/etc/zprofile` and last to `.zshrc`. Without this, running
shell command from within Vim may result in `command not found`.
Merge pull request #85 from AdrienGiboire/master

improv: set shell to ZSH in interactive mode