spf13_vim role fixes

Fixed some issues I had with the spf13_vim role.

The backup commands were failing:
```
TASK: [spf13_vim | backup .vim] ***********************************************
failed: [localhost] => {"changed": true, "cmd": ["mv", "~/.vim", "~/.vim.`date", "+%Y%m%d_%s`"], "delta": "0:00:00.009557", "end": "2014-06-18 20:06:23.877611", "item": "", "rc": 1, "start": "2014-06-18 20:06:23.868054"}
stderr: mv: target ‘+%Y%m%d_%s`’ is not a directory
...ignoring

TASK: [spf13_vim | backup .vimrc] *********************************************
failed: [localhost] => {"changed": true, "cmd": ["mv", "~/.vimrc", "~/.vimrc.`date", "+%Y%m%d_%s`"], "delta": "0:00:00.003468", "end": "2014-06-18 20:06:23.945563", "item": "", "rc": 1, "start": "2014-06-18 20:06:23.942095"}
stderr: mv: target ‘+%Y%m%d_%s`’ is not a directory
...ignoring

TASK: [spf13_vim | backup .gvimrc] ********************************************
failed: [localhost] => {"changed": true, "cmd": ["mv", "~/.gvimrc", "~/.gvimrc.`date", "+%Y%m%d_%s`"], "delta": "0:00:00.003466", "end": "2014-06-18 20:06:24.012892", "item": "", "rc": 1, "start": "2014-06-18 20:06:24.009426"}
stderr: mv: target ‘+%Y%m%d_%s`’ is not a directory
...ignoring
```

And also updated the setup vundle command to match the [upstream bootstrap script](https://github.com/spf13/spf13-vim/blob/9028b86c7f71176f596f5c382e2d9c8e685241e1/bootstrap.sh#L147).