Use RuboCop RSpec 2.26

This commit makes this repo to use RuboCop RSpec 2.26 and suppress the following new offense:

```console
$ bundle exec rubocop

spec/project_spec.rb:263:11: C: [Correctable] RSpec/ExampleWording:
Do not use the future tense when describing your tests.
      it 'will not have a directory' do
          ^^^^^^^^^^^^^^^^^^^^^^^^^
```

Gemfile

@@ -11,7 +11,7 @@ gem 'rake'
 gem 'rspec'
 gem 'rubocop', github: 'rubocop/rubocop'
 gem 'rubocop-performance', '~> 1.20.0'
-gem 'rubocop-rspec', '~> 2.25.0'
+gem 'rubocop-rspec', '~> 2.26.0'
 gem 'simplecov'
 gem 'test-queue'
 gem 'yard', '~> 0.9'

spec/project_spec.rb

@@ -260,7 +260,7 @@
 
       dir = File.expand_path('../changelog', __dir__)
 
-      it 'will not have a directory' do
+      it 'does not have a directory' do
         expect(Dir["#{dir}/*"].none? { |path| File.directory?(path) }).to be(true)
       end