Add a constraint on "shoulda-matchers" version (#17828).

This helps bundler find its dependencies in case shoulda-matchers v1.4.2 is
already installed. In such a case bundler breaks while it shouldn't.


git-svn-id: http://svn.redmine.org/redmine/trunk@13411 e93f8b46-1217-0410-a6f0-8f06a7374b81