(CFACT-137) Fix Unicode support

Command-line arguments aren't necessarily passed in Unicode, Boost.Log and
Boost.Filesystem use wchar_t instead of UTF-8 by default, and Ruby uses
local encoding by default. Change all of these to use UTF-8 in char.

ostringstream wasn't working correctly on Windows, so some logging didn't
appear. Also, for copying a UTF-8 string, we need the #bytesize rather than
(number of characters) #size.

Move testing to Ruby 2.1.5 on Windows to avoid a bug in Ruby's #load
method with Unicode paths, filed as ticket CFACT-170.