Use stable node.js on travis
Use stable node.js on travis

With the gulp4 upgrade, the build should work on node 12 now.

WikiEducationFoundation/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard,WikiEducationFoundation/WikiEduDashboard