Use `candidates-files` instead of `candidates-process` in `helm-source-dictionary`.

This implements Thierry's suggestion to use `candidates-file` instead of `candidates-process` in `helm-source-dictionary` (see issue #9).  Benefits:

- Simpler code and thus perhaps more robust.
- Handles missing dictionary (fixes issue #7).
- No dependency on grep (good for Windows users, I suppose).
- Standard Emacs search semantics (searches are case-insensitive unless there is an upper-case letter in the search term).