Update simplecov gem to 0.16 (#30963).

git-svn-id: http://svn.redmine.org/redmine/trunk@17928 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -79,7 +79,7 @@ end
 group :test do
   gem "rails-dom-testing"
   gem "mocha"
-  gem "simplecov", "~> 0.14.1", :require => false
+  gem "simplecov", "~> 0.16.1", :require => false
   # For running system tests
   gem 'puma', '~> 3.7'
   gem "capybara", '~> 2.13'