weakref: fix deprecation warning after nan upgrade
weakref: fix deprecation warning after nan upgrade

The memory management of the `data` structure is now handled
internally by nan.

KenanSulayman/node-weak,unbornchikken/node-weak,robcolburn/node-weak,TooTallNate/node-weak,fastest963/node-weak,EvolveLabs/electron-weak,robcolburn/node-weak,unbornchikken/node-weak,TooTallNate/node-weak,EvolveLabs/electron-weak,fastest963/node-weak,kkoopa/node-weak,KenanSulayman/node-weak,kkoopa/node-weak,TooTallNate/node-weak,EvolveLabs/electron-weak,fastest963/node-weak,KenanSulayman/node-weak,unbornchikken/node-weak,robcolburn/node-weak,kkoopa/node-weak