Fix edit command on windows

On (at least) mingw32 the manual replacement which is removed by this PR does not work together with Shellwords.split.

Given the path `C:/foo/bar` is returned from `Pry::Editor#temp_file`. When sanitized in the current implementation it is changed to `C:\foo\bar`.

Within `Pry::Editor#open_editor`, `Shellwords.split` cripples this string to `C:foobar`.

This PR changes this to just pass the Posix-style path to windows, which is able to handle slashes on its own. Tested on Windows 2003, Windows 7, Windows 2012