fix support for RuboCop 0.84.0 and 0.85.x

These were the first versions to use rubocop-ast, and unfortunately
the dependency is loose enough that rubocop-ast versions >= 0.7.0
are allowed, which cause require 'rubocop' to fail with

  NoMethodError:
    undefined method `join' for #<Set: {:==, :===, :!=, :<=, :>=, :>, :<}>
  in rubocop-0.84.0/lib/rubocop/cop/style/redundant_conditional.rb:57:in `<class:RedundantConditional>'

This is due to rubocop/rubocop-ast#22
so just block those versions directly. This has the unfortunate
side effect of requiring rubocop-ast for older versions which
don't require it.

pronto-rubocop.gemspec

@@ -34,6 +34,7 @@ Gem::Specification.new do |s|
 
   s.add_runtime_dependency('pronto', '~> 0.11.0')
   s.add_runtime_dependency('rubocop', '>= 0.63.1', '< 1.0')
+  s.add_runtime_dependency('rubocop-ast', '>= 0.6.0', '!= 0.7.0', '!= 0.7.1', '!= 0.8.0')
   s.add_development_dependency('rake', '~> 12.0')
   s.add_development_dependency('rspec', '~> 3.4')
   s.add_development_dependency('rspec-its', '~> 1.2')