Change encoding of filename on cygwin

I used "jekyll" in cygwin, but jekyll build did not work.

When it watched a cause, it was like the problem at the time of judgment of an article generated by _sites once that the name of a file made with Japanese was SJIS(Windows-31J).

Because it seemed to be performed in Cleaner#existing_files, this processing moved when I made the encoding of the file name UTF8 forcibly. As for it, time of Windows and the encoding of the file name are time of the SJIS a value of RUBY_PLATFORM.