Allow failure on some gems on ruby 2.6.0-preview2
Allow failure on some gems on ruby 2.6.0-preview2

google-protobuf is not yet supporting 2.6 version.

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