Updated the "minitest" dependency to 4.2 to prevent dependency hell failures

Example:

--> https://travis-ci.org/elasticsearch/elasticsearch-rails/jobs/34821899#L379

      bundle install --gemfile /home/travis/build/elasticsearch/elasticsearch-rails/elasticsearch-model/gemfiles/4.0.gemfile
      Fetching gem metadata from https://rubygems.org/.........
      Resolving dependencies...
      Bundler could not find compatible versions for gem "minitest":
        In Gemfile:
          activemodel (~> 4) ruby depends on
            activesupport (= 4.0.0) ruby depends on
              minitest (~> 4.2) ruby
          minitest (4.0.0)
      rake aborted!