Add runtime check for task.timeout

This only exist in opal-rspec 0.5.0

Author: zetachang

Rakefile

@@ -20,7 +20,7 @@ Opal::RSpec::RakeTask.new('opal:rspec') do |s, task|
   s.append_path React::Rails::AssetVariant.new(addons: true).react_directory
   s.append_path 'spec/vendor'
   s.index_path = 'spec/index.html.erb'
-  task.timeout = 80000
+  task.timeout = 80000 if task
 end
 
 task :test do