Bump backstopjs image

[tyler36]

This PR bumps the backstopjs image from 6.2.2 to 6.3.3.

This is desirable to remove the deprecation warning displayed when generating references (and probably testing too):

Puppeteer old Headless deprecation warning:
In the near feature 'headless: true' wilt default to the new Headless mode
for Chrome instead of the old Headless irplementation. For more ...