deps: update JerryScript to v2.0

[legendecas]

JerryScript has release v2.0, we should update our deps as soon as possible.

Following is a clip of JerryScript 2.0 release note:

A short list of changes/improvements

• General performance and memory improvements (on ARM RPI3 ~18% speedup and memory usage reduction)
• Binary code size reduction (on ARM RPI3 ~24 kB binary size reduction with ES5.1 profile)
• Minimal API breaking changes (see Migration guide)
• Added new API functions (see API reference document)
• Added debugger support
• Created extension library which includes common use-cases, utility functions
• Added a subset of ES6 (ES2015) features:
  • Arrow functions
  • Class support
  • Module support
  • Rest parameters and enhanced object initializer support
  • Template strings
  • For-of support
  • Symbols
  • Promises
  • Map/Set/Iterator built-ins
  • TypedArray/ArrayBuffer/DataView built-ins
• New experimental target platforms:
  • OpenWrt, Particle, and Windows
• A few statistics (since JerryScript 1.0):
  • 1000+ commits by 73+ authors
  • 1000+ merged Pull requests

Checklists

• TODO