Support LanguageTool JSON API

LT introduced a new HTTP API last year and starting with version 3.6, they no longer support the old one. This means that the lib-verification code (used by CheckMate, etc) will only work with version 3.5 and older.

We need to update to the new API.

More info here: https://languagetool.org/http-api/migration.php

See also this thread: https://groups.yahoo.com/neo/groups/okapitools/conversations/messages/5272