Satidfy a newly rigorous compiler warning

Signed-off-by: Nick Cameron <nrc@ncameron.org>

Author: nrc

src/errors.rs

@@ -95,7 +95,7 @@ pub enum ErrorKind {
 }
 
 impl Fail for Error {
-    fn cause(&self) -> Option<&Fail> {
+    fn cause(&self) -> Option<&dyn Fail> {
         self.inner.cause()
     }