Fixed spf13_vim backup tasks failing

Sub shell date command was not interpreted correctly
Updated spf13_vim setup command to match the one used upstream

See https://github.com/spf13/spf13-vim/blob/9028b86c7f71176f596f5c382e2d9c8e685241e1/bootstrap.sh#L147
Merge pull request #31 from jeanregisser/master

spf13_vim role fixes