Fix appraisal command in CONTRIBUTING.md file

[vsppedro]

Hello, I was following the CONTRIBUTING.md file.

At some point it suggest to run this command:

bundle exec appraisal 5.2 rspec spec/unit/shoulda/matchers/active_model/validate_inclusion_of_matcher_spec.rb

But instead of running the tests, it returned:

bundler: command not found: 5.2

So I tried to see the available list of appraisals by running:

bundle exec appraisal list

And this is what I got:

rails_4_2
rails_5_0
rails_5_1
rails_5_2
rails_6_0

Then I replaced the 5.2 with rails_5_2 and the command worked.

[vsppedro]

Should I start the Travis CI again? I only updated the CONTRIBUTING.md file.

The problem:

1) Shoulda::Matchers::ActiveRecord::ValidateUniquenessOfMatcher when the model has a scoped uniqueness validation when one of the scoped attributes is an integer column when more than one record exists that has the next version of the attribute's value accepts

     Failure/Error: expect(record).to validate_uniqueness.scoped_to(:scope)

     

       Expected Example to validate that :attr is case-sensitively unique

       within the scope of :scope, but this could not be proved.

         Given an existing Example, after setting its :attr to ‹"dummy value"›,

         then making a new Example and setting its :attr to ‹"dummy value"› as

         well and its :scope to a different value, ‹nil›, the matcher expected

         the new Example to be invalid, but it was valid instead.

     Shared Example Group: "it supports scoped attributes of a certain type" called from ./spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb:761

     # ./spec/unit/shoulda/matchers/active_record/validate_uniqueness_of_matcher_spec.rb:64:in `block (4 levels) in <top (required)>'

[guialbuk]

I ran just the failing build again. Everything is fine now.
Thank you for your contribution 🎉