Fix RuboCop Style/SelectByKind and Style/PredicateWithKind offenses

lib/notare/document.rb

@@ -56,7 +56,7 @@ module Notare
     end
 
     def lists
-      @nodes.select { |n| n.is_a?(Nodes::List) }
+      @nodes.grep(Nodes::List)
     end
 
     def uses_lists?

test/document_test.rb

@@ -69,6 +69,6 @@ class DocumentTest < Minitest::Test
     doc.table { doc.tr { doc.td "Cell" } }
 
     assert_equal 2, doc.lists.count
-    assert(doc.lists.all? { |l| l.is_a?(Notare::Nodes::List) })
+    assert(doc.lists.all?(Notare::Nodes::List))
   end
 end