ci: remove node 8 from travis
ci: remove node 8 from travis

This is done duo to Rollup not supporting Node 8 anymore. Here I need to find a different way to
execute tests in a Node 8 environment. For now this will be done locally.

MartinHelmut/number-generator,MartinHelmut/number-generator,MartinHelmut/number-generator,MartinHelmut/number-generator,MartinHelmut/number-generator,MartinHelmut/number-generator