dos2unix plugin/bufexplorer.vim

if dos, vim will complain:

```
Error detected while processing /Users/menghan/codespace/vimfiles/bundle/bufexplorer.zip/plugin/bufexplorer.vim:
line   66:
E492: Not an editor command: <0d>
line   69:
Press ENTER or type command to continue
E15: Invalid expression: exists("g:bufexplorer_version") || &cp<0d>
line 1217:
E171: Missing :endif
"
Press ENTER or type command to continue
```