Update ammeter (to a temporary repo) to support RSpec 4

[pirj]

ammeter is rspec-rails's development dependency, used to test generators.
Its runtime contains code that is not compatible with the latest changes in RSpec 4 (unification of metadata multi-condition filtering), making rspec-core's rspec-rails sub-build to fail.

    c.include Ammeter::RSpec::Rails::GeneratorExampleGroup,
      :type          => :generator,
      :file_path     => lambda { |file_path, metadata|
        metadata[:type].nil? && generator_path_regex =~ file_path
      }

This is a temporary measure to use ammeter's patched fork until the patch is accepted and a newer version is released.

See

• Unify multiple-condition filtering rspec-core#1821
• Unify multi-condition filtering rspec-core#2874
• Support RSpec 4 alexrothenberg/ammeter#64
• example failure (Ammeter::RSpec::Rails::GeneratorExampleGroup that defines destination is not included) https://github.com/rspec/rspec-core/pull/2874/checks?check_run_id=1942170588

[pirj]

Alternatively, we could just add the following to our spec/spec_helper.rb until ammeter is updated:

    c.include Ammeter::RSpec::Rails::GeneratorExampleGroup,
      :type          => :generator
    c.include Ammeter::RSpec::Rails::GeneratorExampleGroup,
      :file_path     => lambda { |file_path, metadata|
        metadata[:type].nil? && generator_path_regex =~ file_path
      }

[pirj]

I like the other approach better. And my suspicion is that we'll have to live longer with the patch, as ammeter doesn't seem to be actively maintained. Depending on my fork of it in the long run would be just weird.