Address png compression issue

[michplunkett]

Describe your changes

There seems to be some issue with the newer versions of pngquant: imagemin/imagemin-pngquant#46 (comment)

Checklist before requesting a review

• All features within the repository still work on my local dev environment.

michaelp@MacBook-Air-5 icj-project-rig % gulp        
`import sass from 'sass'` is deprecated.
Please use `import * as sass from 'sass'` instead.
[16:10:17] Using gulpfile ~/Documents/GitHub/icj-project-rig/gulpfile.js
[16:10:17] Starting 'default'...
[16:10:17] Starting 'clean'...
[16:10:17] Starting 'styles'...
[16:10:17] Starting 'copy'...
[16:10:17] Starting 'nunjucks'...
[16:10:17] Starting 'bake'...
[16:10:17] Finished 'clean' after 4 ms
[16:10:17] Starting 'lint'...
[16:10:17] Starting 'scripts'...
[16:10:17] Starting 'images'...
[16:10:17] Finished 'nunjucks' after 250 ms
[16:10:17] Finished 'bake' after 252 ms
[16:10:17] Finished 'lint' after 463 ms
[16:10:20] Finished 'styles' after 3.27 s
[16:10:20] Finished 'scripts' after 3.02 s
[16:10:20] Finished 'copy' after 3.27 s
[16:10:20] gulp-imagemin: Minified 0 images
[16:10:20] Finished 'images' after 3.03 s
[16:10:20] Starting 'format'...
[16:10:20] File 'docs/books/the-clown.html' was not formatted with Prettier
[16:10:20] File 'docs/books/the-hearing-trumpet.html' was not formatted with Prettier
[16:10:20] File 'docs/books/the-shipping-news.html' was not formatted with Prettier
[16:10:20] File 'docs/index.html' was not formatted with Prettier
[16:10:20] Finished 'format' after 102 ms
[16:10:20] Finished 'default' after 3.38 s
michaelp@MacBook-Air-5 icj-project-rig % 

[critmcdonald]

Works for me. Thanks.