Downgrading Node version to fix travis build
Downgrading Node version to fix travis build

Apparently karma currently does not work in latest version of Node. Read
more here:

https://github.com/karma-runner/karma/pull/1182

Fixes #90

GerHobbelt/refluxjs,simonvizzini/refluxjs,uniphil/refluxjs,mjhlybmwq/refluxjs,VanTanev/refluxjs,williamle8300/refluxjs,Sipree/refluxjs,bsingr/refluxjs,sahat/refluxjs,ericf89/reflux-core,robert-chiniquy/refluxjs,VasylHryha/refluxjs,vebin/refluxjs,watonyweng/refluxjs,yetone/refluxjs,taskworld/refluxjs,insionng/refluxjs,Sipree/reflux-core,reflux/reflux-core,reflux/refluxjs,Mayho/refluxjs,H3Chief/reflux-core,juliankrispel/refluxjs,misnet/refluxjs,lishengzxc/refluxjs,spoike/refluxjs,StephenGrider/refluxjs,goatslacker/refluxjs