Remove old version of FS cache automatically

This will fix the problem found in #121 and #129.

- index.json stores version (int) now.
- cache is stored at `~/.cache/jedi/X.Y/HASH.pkl`, so that you can remove cache for specific Python version X.Y by `shutil.rmtree`.  Otherwise, you will have trouble when using Jedi with different cache version number installed for different Python version.