travis: add Node.js v12, remove v0.12, v4, v6, and v7
travis: add Node.js v12, remove v0.12, v4, v6, and v7

Closes https://github.com/KenanY/secure-random-octet/issues/9
Closes https://github.com/KenanY/secure-random-octet/issues/10
Closes https://github.com/KenanY/secure-random-octet/issues/11
Closes https://github.com/KenanY/secure-random-octet/issues/12
Closes https://github.com/KenanY/secure-random-octet/issues/13

KenanY/secure-random-octet