Lower minitest dependency to >= 4.2.0.

Rails 4.0.0 [requires](https://github.com/rails/rails/blob/v4.0.0/activesupport/activesupport.gemspec#L26) minitest `~> 4.2.0`, which is incompatible with Dalli's `>= 5.0.0` requirement. This is making the Travis tests fail for the rails4 gemfile.