Bump rubocop from 1.14.0 to 1.15.0

Bumps [rubocop](https://github.com/rubocop/rubocop) from 1.14.0 to 1.15.0.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v1.14.0...v1.15.0)

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

Gemfile

@@ -51,7 +51,7 @@ gem 'rdiscount', '~> 2.2'
 gem 'react-rails'
 gem "recaptcha", require: "recaptcha/rails"
 gem 'responders', '~> 3.0'
-gem 'rubocop', '~> 1.14.0', require: false
+gem 'rubocop', '~> 1.15.0', require: false
 gem "ruby-openid", :require => "openid"
 gem 'sanitize'
 gem 'sentry-raven'

Gemfile.lock

@@ -463,7 +463,7 @@ GEM
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.10.0)
     rspec-support (3.10.0)
-    rubocop (1.14.0)
+    rubocop (1.15.0)
       parallel (~> 1.10)
       parser (>= 3.0.0.0)
       rainbow (>= 2.2.2, < 4.0)
@@ -672,7 +672,7 @@ DEPENDENCIES
   rest-client
   reverse_markdown
   rspec
-  rubocop (~> 1.14.0)
+  rubocop (~> 1.15.0)
   ruby-openid
   sanitize
   sassc (~> 2.4.0)