Support node 8 in CKEditor build

The Hippo CKEditor fork on Github (https://github.com/onehippo/ckeditor) does not build with node 8 because some additional plugins (CodeMirror, textselection, wordcount) "require" node 6 in their build. They should also support node 8 so all builds (including CKEditor) can use the same node version (8).