Dompdf security fix

[joewhitsitt]

Resolves #7454

To Test

• See that blt/travis goes through security checks successfully.
• Test admissions pdfs for noticeable issues:
  • ddev blt ds --site=admissions.uiowa.edu && ddev drush @admissions.local uli print/pdf/node/2786
  • Try a few more here admissions.uiowa.ddev.site/areas-of-study/print-links and compare with https://admissions.uiowa.edu/areas-of-study/print-links

[joewhitsitt]

I did test this as part of the issue. Basically looking for a second set of eyes on checking pdf diffs (if any).

[pyrello]

Pulling to test.

[pyrello]

I tested 3 PDFs and could discern no differences. Thanks for the easy test instructions!