Release version 0.1.6

This update mainly just cleans stuff up and updates for the latest rustc (ea6f65c5f)

• Update to using f.write_str(... instead of write!(f, "{}", ...) for simplicity
• Update to use (closure)(...) instead of closure.call((...)) because directly calling works now
• Remove #![feature()] attributes for unboxed_closures and old_orphan_check, as they are no longer required.