Support for node v12

[josmas]

Description of the bug
The library does not install on node v12. Given that v10 is now on maintenance and v12 is the current active, support would be appreciated.

Steps to reproduce

1. Follow instructions and run npm install --save web3c on a machine running node v12.

Expected behavior
The module should install, but instead fails to do so.
Errors include, among many others:

In file included from ../src/node-boilerplate/inc/scrypt_async.h:28:
../src/node-boilerplate/inc/scrypt_common.h:39:63: error: too few arguments to function call, single argument 'context' was not specified
      N(obj->Get(Nan::New("N").ToLocalChecked())->Uint32Value()),
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

Environment

• OS: Both Mac and Docker image for node12
• Versions of relevant software and tools: Empty project just with this library.

Additional information
The package still works (despite the multitude of deprecation warnings) on node v10.

Thanks!