Add test for old version Node

[yunnysunny]

Some version of development packages we used is not suitable for old version node. This packages include eslint commitlint husky remark-cli xo. I moved all these to optional dependencies . And now we can install dependencies via yarn install --ignore-optional to skip the installing of incompatible packages.

And I also found the files in test folder can't been run on old version Node, so I transform them to ES5 files via bable.

[codecov-commenter]

Codecov Report

Merging #1722 (1330634) into master (0b5bbf2) will increase coverage by 0.08%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1722      +/-   ##
==========================================
+ Coverage   86.67%   86.76%   +0.08%     
==========================================
  Files          14       14              
  Lines        1133     1133              
==========================================
+ Hits          982      983       +1     
+ Misses        151      150       -1     

Impacted Files	Coverage Δ
src/node/index.js	93.27% <0.00%> (+0.18%)	:arrow_up:

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[perrin4869]

This broke my CI, I don't think this is a good change. npm will install the optional dependencies by default, even though they really are just dev dependencies. I think the true solution is to drop old node version support

[titanism]

Agreed

[titanism]

v7.1.5 released to npm (note the tests are broken until someone rewrites them)

https://github.com/visionmedia/superagent/releases/tag/v7.1.5

[yunnysunny]

The fix has supplied. https://github.com/visionmedia/superagent/pull/1729