This reverts commit 303c5998f8.
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 8167f8bf54.
