Two failed tests after upgrading 24pr project's shoulda-matchers gem to 3.1.0

gistfile1.txt

If you upgrade the 24pr project's shoulda-matchers gem to 3.1.0,
you get these two failed tests:
```
Failures:

  1) ProjectLabel validations must only have a label allocated to a project once
     Failure/Error: is_expected.to validate_uniqueness_of(:label_id).scoped_to(:project_id)

       ProjectLabel did not properly validate that :label_id is
       case-sensitively unique within the scope of :project_id.
         The record you provided could not be created, as it failed with the
         following validation errors:
         * label_id: ["can't be blank"]

  2) Project should validate that :github_url is case-insensitively unique,
     producing a custom validation error on failure

     Failure/Error:
       it { is_expected.to validate_uniqueness_of(:github_url).
         case_insensitive.with_message('Project has already been suggested.') }

       Project did not properly validate that :github_url is case-insensitively
       unique, producing a custom validation error on failure.
         The record you provided could not be created, as it failed with the
         following validation errors:
         * description: ["can't be blank", "is too short (minimum is 20
           characters)"]
         * github_url: ["can't be blank", "Enter a valid GitHub URL."]
         * name: ["can't be blank"]
         * main_language: ["can't be blank", "must be a programming language"]
```