Fix elm tests

The version resolver fetches the latest version from the public
registry, and since we shell out to `elm` for this, it's currently hard
to stub out.

Our test fixture did not have the latest version yet and would cause our
tests to fail.

Ideally we'd rely on both the version_checker and resolver using
the same source that we stub, but unfortunately with the way our tests
are set up makes that hard. Fortunately these don't change often, so I
suggest we live with the pain for now.

elm/spec/fixtures/elm_package_responses/mercurymedia-elm-datetime-picker.json

@@ -9,5 +9,6 @@
     "3.0.0": 1597843000,
     "4.0.0": 1611232572,
     "4.0.1": 1611308573,
-    "5.0.0": 1618233604
+    "5.0.0": 1618233604,
+    "5.0.1": 1633531907
 }