Clifton: download ældre versioner

Added
- Clifton now supports certificate response version 3.
- Add option to automatically write the SSH config on a successful auth using clifton auth --write-config=true (PR #139). Thanks to David Llewellyn-Jones (@llewelld).
Fixed
- Quote paths in config files. This is required for paths which have spaces in them.
- Allow --config-file to be passed at any level of subcommands.
- Allow clifton clear-cache to work if cache directory is missing.
- Only notify to write config if a change is detected.
- Store the absolute path of passed identity files, rather than their relative position from where auth was run (Issue #98).
Changed
- Retrieve OIDC client ID from the CA rather than setting it in the config. Requires version 0.3 of Conch.
- Make the clifton ssh-command command hidden.

# Changed

- Use the new Conch CA
- Only write the SSH config if the contents has actually changed.
- Shorten the timeout for version check to 5 seconds.