rspec 3.1.2 doesn't accept Rake::FileList for pattern=

Either way, this doesn't have to be explicitly specified since the pattern is
defaulted to 'spec/**{,/*/**}/*_spec.rb' which already covers all spec files
in this project.
Merge pull request #149 from amatsuda/rspec31

rspec 3.1.2 doesn't accept Rake::FileList for pattern=