Implement `file_write' action

spec/producer/core/task/dsl_spec.rb

@@ -6,7 +6,7 @@ module Producer::Core
     let(:env)     { double 'env' }
     subject(:dsl) { Task::DSL.new(&block) }
 
-    %w[echo sh].each do |action|
+    %w[echo sh file_write].each do |action|
       it "has `#{action}' action defined" do
         expect(dsl).to respond_to action.to_sym
       end