Bump browserify-sign to v4.2.3

[EmilyZhang777]

Bump browserify-sign to v4.2.3 to resolve a vulnerability.

J=VULN-38744
TEST=auto,manual

Ran npm run test and npm run acceptance. Ran npx serve -p 9999 and clicked around.

[coveralls]

coverage: 62.02%. remained the same
when pulling b9ea30a on dev/vulnerabilities-2
into 66fef30 on master.

[nmanu1]

did you also check that running the build and using the generated bundles in the Theme test site was fine? it looks like browserify-sign is dep of a library we use for bundling, so it would affect our build step

[nmanu1]

this change should point to a hotfix branch if we're making a new version, or develop if we're not

[EmilyZhang777]

did you also check that running the build and using the generated bundles in the Theme test site was fine? it looks like browserify-sign is dep of a library we use for bundling, so it would affect our build step

yep! Tested HH theme still works with a local build.