Add simplecov and coveralls to Gemfile because Ruby 2.5 seems to need it on Travis CI.

Author: gkellogg

Gemfile

@@ -52,4 +52,6 @@ end
 group :test do
   gem 'equivalent-xml'
   gem 'rake'
+  gem 'simplecov',      '~> 0.16', platforms: :mri
+  gem 'coveralls',      '~> 0.8', '>= 0.8.23',  platforms: :mri
 end