Bump rubocop-rspec from 2.3.0 to 2.4.0

Bumps [rubocop-rspec](https://github.com/rubocop/rubocop-rspec) from 2.3.0 to 2.4.0.
- [Release notes](https://github.com/rubocop/rubocop-rspec/releases)
- [Changelog](https://github.com/rubocop/rubocop-rspec/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop-rspec/compare/v2.3.0...v2.4.0)

---
updated-dependencies:
- dependency-name: rubocop-rspec
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -418,7 +418,7 @@ GEM
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.7.0, < 2.0)
-    rubocop-rspec (2.3.0)
+    rubocop-rspec (2.4.0)
       rubocop (~> 1.0)
       rubocop-ast (>= 1.1.0)
     ruby-progressbar (1.11.0)