It looks like resemble causes the random crashes during UI tests
on Travis CI. I can reproduce this locally and identified resemble
as the problem. The tests crashed because of too much memory usage
We now also avoid loading the images into PhantomJS to compare
them. By executing ImageMagick we seem to save a lot of memory and
time. Eg currently on master the tests need more than 50
minutes whereas with this solution it takes only approx 37 minutes.

Added a background color in the screenshot diff to make it easier to see diffs with white backgrounds

[skip ci]

On local it does not work and I have not figured out why.
Maybe it works on travis though?