Switch to latest mingw-w64

Fixes a ruby-2.0.0-p0 issue. The reason for the removed comment isn't
clear: perhaps it used to fail when supporting 1.8.7?

Closes #40.
Merge pull request #42 from chrismdp/fix-ruby2-mingw-build-exception

Switch to latest mingw-w64 to fix ruby-2.0.0-p0 build