Deprecate the custom exist matcher

[adamstegman]

Prefer the one built-in by RSpec instead. Fall back to the built-in matcher for default behavior when given a filename.

Fixes #60

[alexrothenberg]

@adamstegman I added you as an collaborator here + owner in rubygems in case you want to merge and release this yourself

[pirj]

This loud deprecation broke RSpec's own CI, which is kept warning-free.

It turned out funny that rspec-rails's specs were relying on the Ammeter's implementation of the exist matcher.

     Failure/Error: expect(generator_spec).to exist
       expected "/home/pirj/source/rspec-rails/tmp/spec/generator/posts_generator_spec.rb" to exist but it does not respond to either `exist?` or `exists?`

Most certainly, we just need to clean this up in rspec-rails specs and use Pathname as you suggest.

Is there a canonical way to transform a bunch of existing specs to work without warnings?

      subject { file('spec/models/posts_spec.rb') }

      it { is_expected.to contain /require 'spec_helper'/ }
      it { is_expected.to exist }

What if we change the file helper method to return a Pathname instead of just a string?

        def file relative
-          File.expand_path(relative, destination_root)
+          Pathname.new(File.expand_path(relative, destination_root))

This worked to fix rspec-rails specs.
Just one of Ammeter's own specs fail. Are there any side effects, or is it fine to adjust specs? I can send a PR.

  1) Ammeter::RSpec::Rails::GeneratorExampleGroup an instance of the group working with files should use destination to find relative root file
     Failure/Error: expect(group.file('app/model/post.rb')).to eq "#{path_to_gem_root_tmp}/app/model/post.rb"

       expected: "/home/pirj/source/tmp/app/model/post.rb"
            got: #<Pathname:/home/pirj/source/tmp/app/model/post.rb>

       (compared using ==)

       Diff:
       @@ -1 +1 @@
       -"/home/pirj/source/tmp/app/model/post.rb"
       +#<Pathname:/home/pirj/source/tmp/app/model/post.rb>

to fix it:

- expect(group.file('app/model/post.rb')).to eq "#{path_to_gem_root_tmp}/app/model/post.rb"
+ expect(group.file('app/model/post.rb').to_path).to eq "#{path_to_gem_root_tmp}/app/model/post.rb"

[pirj]

#70

[adamstegman]

Great idea! It feels pretty safe to me in general, but worth calling out as a potential breaking change.