Remove Nodejs 0.12 for Travis-CI and Appveyor

[NoahDragon]

I think Hexo should move forward, and relax some backward compatibility.

Eventually, we should only focus on the Nodejs stable and latest LTS build. Although 0.12 is a stable version, it will soon stop supported.

An additional suggestion is to re-write Hexo in ES2015 standard or maybe TypeScript.

[JLHwung]

lebab will help when re-writting hexo in ES2015.

[NoahDragon]

@JLHwung Great tool!