update rpc call as replace_file_source has been deprecated

[kermorgant]

see phpactor/phpactor#530

Quoting phpactor's author

Now supports generating a diff and applying the text edits
Quite a big change and introduces a new RPC call and deprecates replace_file_source
Also the Emacs integration will have to support update_file_source - but it can simply delegate to the legacy "replace_file_source" for now, as the new call also contains the old response

[kermorgant]

fixed by upcoming #43