Use the new trait object syntax introduced in 1.27.0 (#4)

https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1270-2018-06-21

Author: yvt

src/lib.rs

@@ -35,7 +35,7 @@
 //!     fn log(&self, msg: &str) {}
 //! }
 //! struct LoggerInfo {
-//!     logger: &'static (Logger + Sync)
+//!     logger: &'static (dyn Logger + Sync)
 //! }
 //!
 //! // The methods for working with our currently defined static logger