Upgrade Travis configuration to node.js version 6
Upgrade Travis configuration to node.js version 6

Version 0.10 has reached its end of life and is not maintained anymore.
https://github.com/nodejs/LTS#lts-schedule

Jolg42/opentype.js,Jolg42/opentype.js,nodebox/opentype.js,moyogo/opentype.js,brawer/opentype.js,googlefonts/opentype.js,fpirsch/opentype.js,axkibe/opentype.js,nodebox/opentype.js,moyogo/opentype.js,googlefonts/opentype.js,axkibe/opentype.js,brawer/opentype.js,fpirsch/opentype.js