mk: downgrade to Node 12.15

'npm run literate' does not work in 12.16 due to
standard-things/esm#855

.travis.yml

@@ -2,7 +2,7 @@
 dist: bionic
 
 language: node_js
-node_js: 12
+node_js: '12.15'
 
 addons:
   apt:

README.md

@@ -3,7 +3,7 @@
 [![Travis build status](https://img.shields.io/travis/com/yoursunny/NDNts?style=flat)](https://travis-ci.com/yoursunny/NDNts) [![Coveralls code coverage](https://img.shields.io/coveralls/github/yoursunny/NDNts?style=flat)](https://coveralls.io/github/yoursunny/NDNts) [![GitHub code size](https://img.shields.io/github/languages/code-size/yoursunny/NDNts?style=flat)](https://github.com/yoursunny/NDNts/)
 
 **NDNts** enables [Named Data Networking](https://named-data.net/) in modern web applications.
-It supports latest version of Node.js, as well as modern browsers including Firefox (desktop) and Chrome (desktop, Android, iOS).
+It supports Node.js 12.x, as well as modern browsers including Firefox (desktop) and Chrome (desktop, Android, iOS).
 
 * [NDNts homepage](https://yoursunny.com/p/NDNts/)
 * [#NDNts on Twitter](https://twitter.com/hashtag/NDNts?f=live) for announcements