Remove del

Fix usage of deprecated del command from Express to use new delete instead.

A special notice to the special case we had to add because javascript won't allow delete as an identifier outside of an object scope (eg.: myObj.delete. This restriction forces us to stay with del in our interface since we call it without an object context (eg.: del('/my/path', ...).

Also, improve gitlab CI file to use the alpine version of mocha/nyc container. Thanks to the --no-optional flag that frees us from DTrace compilation (underground issue).