update version of node.js in readme to v6 (LTS) as noted by @rachaelcodes in https://github.com/dwyl/learn-travis/issues/14#issuecomment-327500019
update version of node.js in readme to v6 (LTS) as noted by @rachaelcodes in https://github.com/dwyl/learn-travis/issues/14#issuecomment-327500019

docdis/learn-travis,dwyl/learn-travis