Fix compiler warnings

src/error.rs

         }
     }
 
-    fn cause(&self) -> Option<&StdError> {
+    fn cause(&self) -> Option<&dyn StdError> {
         match *self.0 {
             ErrorKind::Io(ref err) => Some(err),
             ErrorKind::Protobuf{ref err, ..} => Some(err),

src/mmap_blob.rs

 
         match slice.len() {
             0 => return None,
-            1 ... 3 => {
+            1 ..= 3 => {
                 self.last_blob_ok = false;
                 return Some(Err(new_blob_error(BlobError::InvalidHeaderSize)));
             },