Fixed multiplatform compatibility when inherit flag is present.

[hectormenendez]

Tested on windows 10 and MacOS sierra. Node v6.2. This is a followup of #22

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This change is 

[M-Zuber]

Does ['pipe', 'inherit', 'inherit'] not work on windows, instead of using customFds?
Will try and play with this tomorrow.

[hectormenendez]

I tried several combinations with stdio and nothing worked for me on windows, just customFds.

[M-Zuber]

@domarmstrong I am trying to work this out, any chance you could send me a minimized version of you setup with 'debug' for me to test?

[M-Zuber]

@hectormenendez with

npm -v -> 3.9.5
node -v -> 6.2.2

['pipe', 'inherit', 'inherit'] works on windows.

Have a bit more to look into, but hopefully that is the only change needed.