Don't set shell to /bin/sh on Windows. Setting the shell to sh on Windows breaks eg. Vundle.

Signed-off-by: ljani <ljani@server.fake>
Added smart case to the ack command for those using ag.
Merge pull request #393 from teasp00n/3.0

Added smart case to the ack command for those using ag.
Merge pull request #391 from ljani/3.0

Don't set shell to /bin/sh on Windows
Merge remote-tracking branch 'spf13/3.0'
MOD: use literal ack