Remove Node.JS 6.x (Boron) dependencies

End-of-life of Node.js 6.x is April 2019, so remove Node.JS 6.x (Boron) dependencies.

Node.js 6 requirements for gitlab-ce itself was already removed in #54065 (closed) (!24305 (merged)).

cf. https://github.com/nodejs/Release

TODOs

Edited by Takuya Noguchi