travis: remove Node.js v4 and v6, add Node.js v12
travis: remove Node.js v4 and v6, add Node.js v12

Closes https://github.com/KenanY/viete-formula/issues/6
Closes https://github.com/KenanY/viete-formula/issues/7
Closes https://github.com/KenanY/viete-formula/issues/8

KenanY/viete-formula