Switch Travis configuration down to node.js version 4
Switch Travis configuration down to node.js version 4

https://github.com/nodebox/opentype.js/pull/251#issuecomment-272175882

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