Apply explict RSpec/DescribedClass rubocop rule

We settled on using this style in commit 4cbe81a1, but didn't add the
rule enforcing this style and we didn't apply it to existing code.

spec/models/ahoy/data_source_spec.rb

@@ -16,18 +16,18 @@ describe Ahoy::DataSource do
     end
 
     it "works without data sources" do
-      ds = described_class.new
+      ds = Ahoy::DataSource.new
       expect(ds.build).to eq x: []
     end
 
     it "works with single data sources" do
-      ds = described_class.new
+      ds = Ahoy::DataSource.new
       ds.add "foo", Ahoy::Event.where(name: "foo").group_by_day(:time).count
       expect(ds.build).to eq :x => ["2015-01-01", "2015-01-02"], "foo" => [2, 1]
     end
 
     it "combines data sources" do
-      ds = described_class.new
+      ds = Ahoy::DataSource.new
       ds.add "foo", Ahoy::Event.where(name: "foo").group_by_day(:time).count
       ds.add "bar", Ahoy::Event.where(name: "bar").group_by_day(:time).count
       expect(ds.build).to eq :x => ["2015-01-01", "2015-01-02", "2015-01-03"], "foo" => [2, 1, 0], "bar" => [1, 0, 2]