This reverts commit 303c5998f8b747e94ac80e2e02c53b47ded11322.
Reason for revert: I tried enabling `ssim-cie94` by default on
ionic-framework test suite, and it proves to be overly strict for their
usecase.
The option defines a comparator to be used to compare images.
Possible values are `"pixelmatch"` and `"ssim-cie94"`.
Note: This reverts commit 8167f8bf548308ad8c6f1188508aadee84f26023.