Test can not run with ruby2.3.0

[lingceng]

I use ruby 2.3.0 as default version. When I run rake test after bundle install.
Errors happen:

/Users/lingceng/.rvm/rubies/ruby-2.3.0/bin/ruby -I"lib:lib:test" -I"/Users/lingceng/.rvm/gems/ruby-2.3.0/gems/rake-10.5.0/lib" "/Users/lingceng/.rvm/gems/ruby-2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb" "test/**/*_test.rb"
/Users/lingceng/.rvm/gems/ruby-2.3.0/gems/shoulda-matchers-1.0.0/lib/shoulda/matchers/assertion_error.rb:4:in `require': cannot load such file -- minitest/unit (LoadError)
        from /Users/lingceng/.rvm/gems/ruby-2.3.0/gems/shoulda-matchers-1.0.0/lib/shoulda/matchers/assertion_error.rb:4:in `<module:Matchers>'

I change to ruby2.1.5, tests passed.