Sublime is not happy about Windows backslashes in resource paths

Without this change Sublime Text is rendered broken shortly after Dart package installation.

What happens is that dartlint.py writes Windows path with backslashes in color_scheme property during initialization(in check_theme), but then sublime.load_resource(theme) can't work when those backslashes in theme path.

The fix is to force forward slashes in the path by using posixpath even on Windows.