remove deprecate warning on Windows

HighLine is using the Win32API, but Win32API is obsolete.
(Win32API = DL Wrapper)
And, DL is deprecated after Ruby 2.0.0.

case 1: use Ruby 2.0.0 : `DL is deprecated, please use Fiddle`
case 2: use Ruby 1.9.3 : `Win32API is deprecated after Ruby 1.9.1; use dl directly instead`
Merge pull request #71 from whiteleaf7/master

Remove deprecate warning on Windows