Wir verwenden Features von neueren PostgreSQL Versionen.

Dies sollte den folgenden Fehler auf Travis CI beheben:

```
PG::UndefinedObject: ERROR:  type "json" does not exist
LINE 1: ...aracter varying(255), "kommentar" text, "payload" json DEFAU...
                                                             ^
: CREATE TABLE "protokoll_eintraege" ("id" serial primary key, "geschaeft_id" integer NOT NULL, "aktion" character varying(255) NOT NULL, "aktion_text" character varying(255), "kommentar" text, "payload" json DEFAULT '{}' NOT NULL, "created_at" timestamp, "updated_at" timestamp) /home/travis/build/4teamwork/geschaeftsverzeichnis/vendor/bundle/ruby/2.0.0/gems/activerecord-4.1.0.beta1/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `exec'
```