Adds the coverage report and coverage badge

googleauth.gemspec

@@ -33,6 +33,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'signet', '~> 0.6.0'
 
   s.add_development_dependency 'bundler', '~> 1.7'
+  s.add_development_dependency 'simplecov', '~> 0.9.2'
   s.add_development_dependency 'rake', '~> 10.0'
   s.add_development_dependency 'rubocop', '~> 0.28.0'
   s.add_development_dependency 'rspec', '~> 3.0'