relax dependent version of multi_json

see also: https://github.com/intridea/multi_json/blob/master/CHANGELOG.md#1111

googleauth.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'logging', '~> 2.0'
   s.add_dependency 'jwt', '~> 1.4'
   s.add_dependency 'memoist', '~> 0.12'
-  s.add_dependency 'multi_json', '1.11'
+  s.add_dependency 'multi_json', '~> 1.11'
   s.add_dependency 'signet', '~> 0.6'
 
   s.add_development_dependency 'bundler', '~> 1.9'