Add node version 6 to travis.
Add node version 6 to travis.

Koodinviilaus/YleFiilis,genarocolusso/zikazero,codeinthewoods/now-playing-yle,aoimedia/web-starter-kit,JonasAymoz/jonasaymoz.github.io,petermodest/transparency,rlugojr/web-starter-kit,codeinthewoods/now-playing-yle,google/web-starter-kit,zdrawku/dance-events,genarocolusso/zikazero,lianliu/Portfolio,zdrawku/dance-events,google/web-starter-kit,rlugojr/web-starter-kit,petermodest/transparency,aoimedia/web-starter-kit,lianliu/Portfolio,JonasAymoz/jonasaymoz.github.io,Koodinviilaus/YleFiilis