ensure .gitignore ends in a newline

If .gitignore doesn't end in a newline then appending to it won't work well.  I discovered this when I ran `rails g half_pipe:install`, then ran `rails g half_pipe:install --processor=less` to correct it.

I ended up with this in my .gitignore:

```
/log/*.log
/tmp
node_modules
bower_components
public/assets/*
!public/assets/imagesnode_modules                           <-- missing newline
bower_components
public/assets/*
!public/assets/images
```

Of course, it would be nice if the generator didn't add lines that are already in the file.