Upgrade standard

It turns out that in lib/ulid/rails/postgresql_type.rb we have been using an
endless range for a while now. Since no bug has been reported, I guess we don't
have any users running Ruby < 2.6.

.standard.yml

@@ -1,4 +1,4 @@
-ruby_version: 2.3
+ruby_version: 2.6
 ignore:
   - 'vendor/**/*'
   - 'gemfiles/vendor/**/*'

ulid-rails.gemspec

@@ -5,7 +5,7 @@ require "ulid/rails/version"
 Gem::Specification.new do |spec|
   spec.name = "ulid-rails"
   spec.version = ULID::Rails::VERSION
-  spec.required_ruby_version = ">= 2.5.0"
+  spec.required_ruby_version = ">= 2.6.0"
   spec.authors = ["Kazunori Kajihiro", "Zendesk"]
   spec.email = ["[email protected]", "[email protected]"]
 
@@ -47,5 +47,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake"
   spec.add_development_dependency "minitest", "~> 5.0"
   spec.add_development_dependency "rubocop-minitest"
-  spec.add_development_dependency "standard", "~> 1.16.0"
+  spec.add_development_dependency "standard", "~> 1.32.0"
 end