node: fix leaking Context handle

The call to node::Environment::GetCurrent(Isolate*) makes the call to
v8::Isolate::GetCurrentContext(). Doing so creates a new handle that
bubbled to the v8::SealHandleScope().

PR-URL: https://github.com/nodejs/node/pull/3945


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: James Snell <jasnell@gmail.com>