Declare simplecov as direct dependency

The undercover docs recommended to remove it from the Gemfile but that's
only valid if you use only undercover. We do rely directly on the
simplecov gem to generate reports though.

Gemfile

@@ -172,6 +172,7 @@ end
 group :test do
   gem 'pdf-reader'
   gem 'rails-controller-testing'
+  gem 'simplecov', require: false
   gem 'undercover', require: false
   gem 'vcr', require: false
   gem 'webmock', require: false

Gemfile.lock

@@ -990,6 +990,7 @@ DEPENDENCIES
   shoulda-matchers
   sidekiq
   sidekiq-scheduler
+  simplecov
   spreadsheet_architect
   spring
   spring-commands-rspec