Fix 'bundle exec rake (spec)' so that it works

Author: mcmire

Rakefile

@@ -1,9 +1,16 @@
-require "rspec/core/rake_task"
 require "bundler/gem_tasks"
 
 require_relative "support/current_bundle"
 
-RSpec::Core::RakeTask.new(:spec)
+begin
+  require "rspec/core/rake_task"
+  RSpec::Core::RakeTask.new(:spec)
+rescue LoadError
+  task :spec do
+    appraisal = SuperDiff::CurrentBundle.instance.latest_appraisal
+    exec "appraisal install && appraisal #{appraisal.name} rake spec --trace"
+  end
+end
 
 task :default do
   if SuperDiff::CurrentBundle.instance.appraisal_in_use?