Fix LocateFile_ag error while Bundling with Vundle

When running `:BundleInstall` from Vundle to install ag.vim, this error
is thrown:

```
Error detected while processing
/Users/david/Documents/Code/dotfiles/vim/bundle/ag/plugin/ag.vim:
line  299:
E122: Function LocateFile_ag already exists, add ! to replace it
```

So, defined LocateFile_ag using `function!` instead.

Signed-off-by: David Celis <me@davidcel.is>