Ensure credit_spec fails

For the tests to fail as expected, the user and organisation test subjects
need to be created after their related credits. To ensure this, I have used
the eager loading `let!`.

spec/models/credit_spec.rb

@@ -9,8 +9,8 @@
   it { is_expected.to belong_to(:purchase).optional }
 
   context "when caching counters" do
-    let(:user_credits) { create_list(:credit, 2, user: user) }
-    let(:org_credits) { create_list(:credit, 1, organization: organization) }
+    let!(:user_credits) { create_list(:credit, 2, user: user) }
+    let!(:org_credits) { create_list(:credit, 1, organization: organization) }
 
     describe "#credits_count" do
       it "counts credits for user" do