Maintenance: Bump rubocop-rails from 2.15.0 to 2.15.1

Bumps [rubocop-rails](https://github.com/rubocop/rubocop-rails) from 2.15.0 to 2.15.1.
- [Release notes](https://github.com/rubocop/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rubocop/rubocop-rails/compare/v2.15.0...v2.15.1)

Gemfile.lock

@@ -477,7 +477,7 @@ GEM
     rubocop-performance (1.14.2)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.15.0)
+    rubocop-rails (2.15.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.7.0, < 2.0)