Bump rubocop-rails from 2.7.1 to 2.8.1 (#758)

Bumps [rubocop-rails](https://github.com/rubocop-hq/rubocop-rails) from 2.7.1 to 2.8.1.
- [Release notes](https://github.com/rubocop-hq/rubocop-rails/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop-rails/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rails@v2.7.1...v2.8.1)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -48,7 +48,7 @@ group :development, :test do
   gem 'byebug', platform: :mri
   gem 'rubocop', '~> 0.92.0', require: false
   gem 'rubocop-performance', '~> 1.8.1'
-  gem 'rubocop-rails', '~> 2.7.1'
+  gem 'rubocop-rails', '~> 2.8.1'
   gem 'dotenv-rails'
 end
 

Gemfile.lock

@@ -267,7 +267,7 @@ GEM
     rubocop-performance (1.8.1)
       rubocop (>= 0.87.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.7.1)
+    rubocop-rails (2.8.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 0.87.0)
@@ -368,7 +368,7 @@ DEPENDENCIES
   resque-heroku-signals
   rubocop (~> 0.92.0)
   rubocop-performance (~> 1.8.1)
-  rubocop-rails (~> 2.7.1)
+  rubocop-rails (~> 2.8.1)
   sassc-rails (~> 2.1.2)
   selenium-webdriver
   sentry-raven