Re-enable phantomJS SSL error reporting in tests.

This was disabled when preview didn't have a valid SSL cert. This is on
longer an issue because 1. preview has a valid SSL cert, and 2. The
tests no longer point at preview[1].

This will hopefully reveal some more meaningful errors when loading
assets fails.

[1] https://github.com/alphagov/slimmer/blob/master/lib/slimmer/test_templates/header_footer_only.html#L19-L21