Bug 1565359 - Fix trait objects without an explicit `dyn` are deprecated error. r=ato

Differential Revision: https://phabricator.services.mozilla.com/D37771

--HG--
extra : moz-landing-system : lando

testing/mozbase/rust/mozversion/src/lib.rs

@@ -263,7 +263,7 @@ impl error::Error for Error {
         }
     }
 
-    fn cause(&self) -> Option<&error::Error> {
+    fn cause(&self) -> Option<&dyn error::Error> {
         match *self {
             Error::SemVerError(ref e) => Some(e),
             Error::VersionError(_) | Error::MetadataError(_) => None,