Fix issue with submodules

```
dotfiles[master]/
    ↪ git submodule init
    No submodule mapping found in .gitmodules for path 'vim/bundle/ctags.vim'
```

Git requires a `.gitmodules` file for submodules, but these aren’t
needed as `vundle` handles finding and cloning vim plugins.

There’s also not really any benefit to locking down a version as vim
plugins don’t have a standardized versioning system. Many don’t have
version numbers at all.

I've added a `.gitkeep` file, which is something I generally don’t like
having, but we need to have the directory around for vundle. I’d be up
for removing that and creating the directory in `install.sh`. Thoughts?