Update premailer gem to v1.16.0

premailer v1.16.0 dropped the use of MD5 in favor of SHA256, which is needed for FIPS compliance. In addition, there have been a number of important fixes: https://github.com/premailer/premailer/blob/master/CHANGELOG.md

Diff: https://my.diffend.io/gems/premailer/1.11.1/1.16.0