Add rake task to load sdg

This task should be useful for existing installations that are going
to upgrade the app and want to load SDG data into an already
existing database.

spec/lib/tasks/load_sdg_spec.rb

@@ -0,0 +1,28 @@
+require "rails_helper"
+
+describe "rake db:load_sdg" do
+  before { Rake::Task["db:load_sdg"].reenable }
+
+  let :run_rake_task do
+    Rake.application.invoke_task("db:load_sdg")
+  end
+
+  it "populates empty databases correctly" do
+    SDG::Goal.destroy_all
+
+    run_rake_task
+
+    expect(SDG::Goal.count).to eq 17
+  end
+
+  it "does not create additional records on populated databases" do
+    expect(SDG::Goal.count).to eq 17
+
+    goal_id = SDG::Goal.last.id
+
+    run_rake_task
+
+    expect(SDG::Goal.count).to eq 17
+    expect(SDG::Goal.last.id).to eq goal_id
+  end
+end