jdutil/spree — comment on commit d055ec0e98a8b0395e1b34a3dd5bc21ab0568cd0
File:  line 0
By jdutil:

They are listed as development dependencies, which are optional for testing purposes. An extension author can update the gemspec as they wish if they would like to change their testing framework. This also properly lists your gems dependencies on sites like rubygems.org, which doesn't take into consideration your projects gemfile.