复制粘体错误到google。找到解决方案:
https://github.com/thoughtbot/shoulda-matchers/issues/1057
GIT remote: https://github.com/thoughtbot/shoulda-matchers.git revision: f800d62525a65f9bf001ea984ae4926a92ff5b82 branch: rails-5 #改为master,因为官方把分支合并了。