Make prism dependency optional

Author: st0012

Gemfile

@@ -10,4 +10,5 @@ group :development do
   gem 'test-unit-ruby-core'
   gem 'rubocop', '>= 1.31.0'
   gem 'gettext'
+  gem 'prism', '>= 0.30.0'
 end

rdoc.gemspec

@@ -230,9 +230,8 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   s.rdoc_options = ["--main", "README.rdoc"]
   s.extra_rdoc_files += s.files.grep(%r[\A[^\/]+\.(?:rdoc|md)\z])
 
-  s.required_ruby_version = Gem::Requirement.new(">= 2.7.0")
+  s.required_ruby_version = Gem::Requirement.new(">= 2.6.0")
   s.required_rubygems_version = Gem::Requirement.new(">= 2.2")
 
-  s.add_dependency 'prism', '>= 0.30.0'
   s.add_dependency 'psych', '>= 4.0.0'
 end