Fix path to orgmode-store.bin.gz

sublime.packages_path() returns a directory, there's no need to use dirname on it:

```pycon
>>> sublime.packages_path()
'/Users/jiffyclub/Library/Application Support/Sublime Text 3/Packages'
```

os.path.join is a better way to construct system paths, abspath ensures we have an absolute path.

I think this should fix #38, though I'm not totally sure how to trigger the error in the first place. Note that I'm using a Mac so I don't know if this could break other systems.