Fixes a lint error in the gemspec

googleauth.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |s|
   s.email         = 'temiola@google.com'
   s.homepage      = 'https://github.com/google/google-auth-library-ruby'
   s.summary       = 'Google Auth Library for Ruby'
-  s.license       = "Apache-2.0"
+  s.license       = 'Apache-2.0'
   s.description   = <<-eos
    Allows simple authorization for accessing Google APIs.
    Provide support for Application Default Credentials, as described at