use explicit dyn with Write to appease compiler

src/output.rs

@@ -98,7 +98,7 @@ impl OutputType {
         OutputType::Stdout(io::stdout())
     }
 
-    pub fn handle(&mut self) -> Result<&mut Write> {
+    pub fn handle(&mut self) -> Result<&mut dyn Write> {
         Ok(match *self {
             OutputType::Pager(ref mut command) => command
                 .stdin