add dependency gem rb-fsevent~> 0.9.1

To run 
$ bundle exec guard

[Listen warning]:
  Missing dependency 'rb-fsevent' (version '~> 0.9.1')!
  Please add the following to your Gemfile to satisfy the dependency:
    gem 'rb-fsevent', '~> 0.9.1'
  
  For a better performance, it's recommended that you satisfy the missing dependency.