Change fileformat to unix.

I'm getting an error, due to the fact that the file is saved as a DOS file, in combination with some line endings. In this pull request I've changed it to the Unix file format to remove the errors listed below:

```
Error detected while processing /Users/case/.vim/bundle/bufexplorer.zip/plugin/bufexplorer.vim:
line   66:
E492: Not an editor command: ^M
line   69:
E15: Invalid expression: exists("g:bufexplorer_version") || &cp^M
line 1217:
E171: Missing :endif
line   66:
E492: Not an editor command: ^M
line   69:
E15: Invalid expression: exists("g:bufexplorer_version") || &cp^M
line 1217:
E171: Missing :endif
Press ENTER or type command to continue
```