You have to setup a C development tool on Window, in order to build 
native extension for Ruby.

You can either use GNU C or Visual C++.

I have the same error on my Window machine. I install Visual C++, and 
afterward the error is solved.

Several users on Linux, Unix or Mac OS have the same problems, because 
they don't install C compiler tools yet. Just use the package management 
to install appropriate C compiler for each environment, it will be Ok.

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