Reset ENV after each spec

  - In certain test run orders, ENV['RESQUTILS_SECONDS_TO_BE_CONSIDERED_STALE']
    was left in an unexpected state with a value like 0, and this value
    would cause subsequent tests to fail.
  - #6 (comment)

spec/stale_workers_spec.rb

@@ -14,8 +14,11 @@
       ]
     }
 
-    before do
+    after do
       ENV.delete("RESQUTILS_SECONDS_TO_BE_CONSIDERED_STALE")
+    end
+
+    before do
       allow(Resque).to receive(:workers).and_return(workers)
     end