removes Minitest caveat from README.md
removes Minitest caveat from README.md

rashmiagar/exercism,wobh/xruby,dalexj/xruby,NobbZ/xruby,samilokan/xruby,NeimadTL/ruby,remcopeereboom/xruby,jdsayle/xruby,dalexj/xruby,Insti/xruby,mamenama/xruby,darrencauthon/xruby,tommyschaefer/xruby,alexbrinkman/xruby,Insti/exercism-ruby,NeimadTL/ruby,exercism/xruby,exercism/xruby,Calvyn82/word_count,gchan/xruby,Calvyn82/xruby,mamenama/xruby,kickinbahk/xruby,NeimadTL/ruby,Insti/xruby,kotp/xruby,darrencauthon/xruby,kotp/xruby,jdsayle/xruby,remcopeereboom/xruby,gchan/xruby,alexbrinkman/xruby,tommyschaefer/xruby,Insti/exercism-ruby,Calvyn82/xruby,rashmiagar/exercism,wobh/xruby,Calvyn82/word_count,samilokan/xruby,NobbZ/xruby,kickinbahk/xruby,Insti/exercism-ruby