Update Bundler version

Author: evolve75

rubytree.gemspec

@@ -66,7 +66,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'json', '~> 2.0', '> 2.9'
 
   # NOTE: Rake is added as a development and test dependency in the Gemfile.
-  s.add_development_dependency 'bundler', '~> 2.3'
+  s.add_development_dependency 'bundler', '~> 2.6'
   s.add_development_dependency 'rake', '~> 13.2'
   s.add_development_dependency 'rdoc', '~> 6.10'
   s.add_development_dependency 'rspec', '~> 3.0', '>= 3.13'