Test under Ruby 2.5.0
Test under Ruby 2.5.0

Still considering 2.4.1 is the main target for database (adapter) and mutation
tests.

Closes: https://github.com/RailsEventStore/rails_event_store/issues/178

RailsEventStore/rails_event_store,arkency/rails_event_store,arkency/rails_event_store,mpraglowski/rails_event_store,arkency/rails_event_store,RailsEventStore/rails_event_store,mpraglowski/rails_event_store,RailsEventStore/rails_event_store,mpraglowski/rails_event_store,RailsEventStore/rails_event_store,mpraglowski/rails_event_store,arkency/rails_event_store