Fix enum/int conversion error with 10.10 SDK

10.10 SDK makes NSLineBreakMode into a proper NS_ENUM enum (in 10.9 SDK it was
a plain typedef for NSUInteger with some integer NSLineBreak* values
conveniently available for it in an anonymous enum).

Nevertheless, attr_string code should have always used NSLineBreakMode instead
of NSUInteger (unless I am missing something like values in style::line_break::mode
that do not correspond to the enum), so this commit makes it so and
thus also fixes building with 10.10 SDK.