remove explicit dependency on bundler

bump required ruby version to >= 2.6, which includes
bundler by default with ruby installs

licensed.gemspec

@@ -21,13 +21,12 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version = ">= 2.3.0"
+  spec.required_ruby_version = ">= 2.6.0"
 
   spec.add_dependency "licensee", ">= 9.15.2", "< 10.0.0"
   spec.add_dependency "thor", ">= 0.19"
   spec.add_dependency "pathname-common_prefix", "~> 0.0.1"
   spec.add_dependency "tomlrb", ">= 1.2", "< 3.0"
-  spec.add_dependency "bundler", ">= 1.10"
   spec.add_dependency "ruby-xxHash", "~> 0.4"
   spec.add_dependency "parallel", ">= 0.18.0"
   spec.add_dependency "reverse_markdown", ">= 1", "< 3"