Fix/windows

[jacobheun]

There is an issue in node 10 where an extra, undefined value is being passed into the fs callbacks. This causes the async methods to fail.

I found the issue on a jenkins build of windows for js-ipfs. I added windows and node 10 to the travis file for this repo to verify the failure, and then added the fix.

You can see the failure on my fork here with windows node 10 added to travis.
And the fixed build here.

[codecov-io]

Codecov Report

Merging #3 into master will decrease coverage by 0.66%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   64.58%   63.91%   -0.67%     
==========================================
  Files           3        3              
  Lines          96       97       +1     
==========================================
  Hits           62       62              
- Misses         34       35       +1

Impacted Files	Coverage Δ
src/portable.js	34.78% <33.33%> (-0.78%)	⬇️

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[dignifiedquire]

Thank you

[dignifiedquire]

published new version