[INTERNAL] Travis CI: Update node version to v6
[INTERNAL] Travis CI: Update node version to v6

npm test expects node v6+ to be used.

Fixes: https://github.com/SAP/openui5/issues/1987
Resolves: https://github.com/SAP/openui5/pull/1986
Change-Id: Ia4ff17f1a4b7cf9a93ba30ae87555bfc441ccab2

SAP/openui5,SAP/openui5,cschuff/openui5,cschuff/openui5,cschuff/openui5,SAP/openui5,SAP/openui5