instruct brew to install opencv@2

[shamrin]

See also:

• issue Compile in Node.js v8 fails. #542
• Compile in Node.js v8 fails. #542 (comment)

[codecov-io]

Codecov Report

Merging #555 into master will increase coverage by 5.62%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   51.19%   56.82%   +5.62%     
==========================================
  Files          34       35       +1     
  Lines        3899     5702    +1803     
  Branches       27       54      +27     
==========================================
+ Hits         1996     3240    +1244     
- Misses       1903     2462     +559

Impacted Files	Coverage Δ
src/Calib3D.cc	5.37% <0%> (-1.75%)	⬇️
src/ImgProc.cc	37.5% <0%> (-0.82%)	⬇️
src/CamShift.cc	90.57% <0%> (-0.54%)	⬇️
src/LDAWrap.cc	73.23% <0%> (-0.1%)	⬇️
src/Features2d.cc	100% <0%> (ø)	⬆️
src/init.cc	100% <0%> (ø)	⬆️
src/CamShift.h	100% <0%> (ø)	⬆️
src/Constants.cc	100% <0%> (ø)	⬆️
src/VideoWriterWrap.h	0% <0%> (ø)	⬆️
src/BackgroundSubtractor.h	0% <0%> (ø)	⬆️
... and 14 more

---

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[kimown]

This PR should merged to master branch, the README.md is outdated.