Fix critical security vulnerabilities

[karlhorky]

Hi there, first of all, thanks for this project! Nice to see work being done in the email ecosystem :)

There are a number of security vulnerabilities in the old dependencies here.

This pull request fixes some of the easier ones (fixed via npm audit fix) and also the critical vulnerability in open.

This is also affecting other projects which depend on heml, such as this one where I have been contributing:

https://github.com/Jinksi/netlify-cms-react-starter/pull/26#issuecomment-580680410

cc @jgzamora @avigoldman @shrirupa @gfriedmansp @nicklemmon

[karlhorky]

cc @jgzamora @avigoldman @shrirupa @gfriedmansp @nicklemmon