Fix crash running machine learning with no data dir

The scripts crashed when the `data` folder wasn't present, which is the
common situation in development environments or production environments
not using Capistrano, since this folder isn't under version control.

app/models/machine_learning.rb

@@ -195,17 +195,24 @@ class MachineLearning
 
   private
 
+    def create_data_folder
+      FileUtils.mkdir_p DATA_FOLDER
+    end
+
     def export_proposals_to_json
+      create_data_folder
       filename = DATA_FOLDER.join(MachineLearning.proposals_filename)
       Proposal::Exporter.new.to_json_file(filename)
     end
 
     def export_budget_investments_to_json
+      create_data_folder
       filename = DATA_FOLDER.join(MachineLearning.investments_filename)
       Budget::Investment::Exporter.new(Array.new).to_json_file(filename)
     end
 
     def export_comments_to_json
+      create_data_folder
       filename = DATA_FOLDER.join(MachineLearning.comments_filename)
       Comment::Exporter.new.to_json_file(filename)
     end

spec/models/machine_learning_spec.rb

@@ -303,9 +303,6 @@ describe MachineLearning do
 
   describe "#export_proposals_to_json" do
     it "creates a JSON file with all proposals" do
-      require "fileutils"
-      FileUtils.mkdir_p Rails.root.join("public", "machine_learning", "data")
-
       first_proposal = create(:proposal)
       last_proposal = create(:proposal)
 
@@ -332,9 +329,6 @@ describe MachineLearning do
 
   describe "#export_budget_investments_to_json" do
     it "creates a JSON file with all budget investments" do
-      require "fileutils"
-      FileUtils.mkdir_p Rails.root.join("public", "machine_learning", "data")
-
       first_budget_investment = create(:budget_investment)
       last_budget_investment = create(:budget_investment)
 
@@ -359,9 +353,6 @@ describe MachineLearning do
 
   describe "#export_comments_to_json" do
     it "creates a JSON file with all comments" do
-      require "fileutils"
-      FileUtils.mkdir_p Rails.root.join("public", "machine_learning", "data")
-
       first_comment = create(:comment)
       last_comment = create(:comment)