Doesn't build against node 7

[chearon]

Issue or Feature

Ran into this doing the prebuilds. 2.2.0 doesn't build in Node 7 on any of the 3 platforms. Node 6 does. Should we add all versions to the build matrix?

Steps to Reproduce

• npm install in the repo
• src/CanvasRenderingContext2d.cc:1722:49: error: no member named 'InstanceOf' in 'v8::Object'

Looks like it was caused by #1293 (this line). Should it be > 7 instead?

Your Environment

• Version of node-canvas (output of npm list canvas or yarn list canvas): 2.2.0
• Environment (e.g. node 4.2.0 on Mac OS X 10.8): any

[chearon]

BTW still none of the 2.x build on node 6 / macOS either, although I've never tried to find out why:

ld: library not found for -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[Hakerh400]

Should it be > 7 instead?

Seems correct. I'll open PR.