Version 1.6.0 with Node.js >= 8.X support

Updated minimum requirements to the project:

• C++ toolset:
  • Microsoft Visual C++ 2015/2017 (Windows 10)
  • GCC 5.4.0 (Ubuntu 16.04)
  • Clang 5.0.0 (Ubuntu 16.04)
• V8 version: >= 6.3
• Node.js version >= 8.0

Fixed issues: #80, #84, #88, #91