Restrict rspec-mutant to 0.8.11
Restrict rspec-mutant to 0.8.11

0.8.13 fails on ruby 2.1.10 with message:
```
warning: parser/current is loading parser/ruby21, which recognizes
warning: 2.1.8-compliant syntax, but you are running 2.1.10.
```

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