Fix error when using Appraisal + Combustion + install_if

Failure/Error: Combustion.initialize! :active_record, :action_controller

SyntaxError:
  /home/nicolas/.asdf/installs/ruby/3.1.2/lib/ruby/gems/3.1.0/bundler/gems/appraisal-40956d3ae826/lib/appraisal/gemfile.rb:33: syntax error, unexpected `end', expecting end-of-input

Instead of using a raw String as explained here (thoughtbot#176) it uses the
interpreted version:

<Proc:0x00007f7aab911770 /home/nicolas/PROJECTS/CONCERTO/gems/ajax-datatables-rails/gemfiles/rails_6.1.4.gemfile:8 (lambda)>
<Proc:0x00007f7aab9177d8 /home/nicolas/PROJECTS/CONCERTO/gems/ajax-datatables-rails/gemfiles/rails_6.1.4.gemfile:12 (lambda)>
<Proc:0x00007f7aab917490 /home/nicolas/PROJECTS/CONCERTO/gems/ajax-datatables-rails/gemfiles/rails_6.1.4.gemfile:16 (lambda)>
"-> { ENV[\"DB_ADAPTER\"] == \"sqlite3\" }"
"-> { ENV[\"DB_ADAPTER\"] == \"mysql2\" }"
"-> { ENV[\"DB_ADAPTER\"] == \"oracle_enhanced\" }"

lib/appraisal/conditional.rb

@@ -14,6 +14,8 @@ def to_s
 
     # :nodoc:
     def for_dup
+      return unless @condition.is_a?(String)
+
       "install_if #{@condition} do\n#{indent(super)}\nend"
     end
   end