Fix not to use deprecated method

Hi,

Currently, `jedi.vim` and `jedi_vim.py` are using deprecated methods.
I've fixed to use new apis.

When using `Vim` + `Python 2.6`, Vim echoes following messages.
(`Python 2.7` did not echo messages)
```
jedi-vim/jedi/jedi/api.py:209: DeprecationWarning: Use completions instead.
  warnings.warn("Use completions instead.", DeprecationWarning)
```

Please check and include it.

Regards,