Added a condition to check for  when assigning cmd to execute.

Currently vundle does not consult the `shell` variable in vim to make changes of its commands. Vundle broke when I used gvim with cygwin's bash as my shell. 

        set shell=C:/cygwin/bin/bash

For example, it would try to use `rmdir` with `/Q` flag when I tried to do BundleClean. The proposed version checks to see if `shell` contains any string match for "sh". This would avoid using Windows only commands for most Unix based shells (e.g. fish, bash, zsh, csh...)