npm install does not run postinstall

[justinbaltazar]

Describe the bug
npm install does not run postinstall script on some versions of NPM. As a result, grammar.js is not generated and Haraka complains that the file does not exist.

Steps To Reproduce
Run npm install in the root directory of this repo as root on some versions of NPM (mine is 6.14.4).

System Info:

Haraka	Haraka.js — Version: 2.8.26
Node	v10.19.0
OS	Linux mail-production-1 5.4.0-1029-oracle #31-Ubuntu SMP Wed Oct 21 13:50:19 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
openssl	OpenSSL 1.1.1f 31 Mar 2020

NPM version: 6.14.4

Additional Context:
One way that we could resolve this is just by modifying the instructions to have the user manually run npm run postinstall. I'm unsure how we could modify this though to run postinstall scripts when installing Haraka from npm.

[msimerson]

The postinstall hook is documented in the v6 docs so I expected it to work for every npm v6+. Silly me.

I just tested installing it on ubuntu installed via nodesource and Haraka and all dependencies installed correctly.

Haraka	Haraka.js — Version: 2.8.26
Node	v10.23.1
OS	Linux ubuntu 4.15.0-128-generic #131-Ubuntu SMP Wed Dec 9 06:57:35 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
openssl	OpenSSL 1.1.1 11 Sep 2018

Since:

- Haraka specifically requires node >= `10.21.0`
- Haraka will soon specifically require >= 10.23.1
- your version of node.js is older than the minimum
- node 10 goes EOL in a few months

I'm not inclined to try very hard to support legacy npm. The solution is for you to update your version of node.js.