Resolve default_trait_access clippy lint in PR 158

    error: calling `anyhow::Chain::default()` is more clear than this expression
      --> tests/test_chain.rs:54:24
       |
    54 |     let mut c: Chain = Default::default();
       |                        ^^^^^^^^^^^^^^^^^^ help: try: `anyhow::Chain::default()`
       |
       = note: `-D clippy::default-trait-access` implied by `-D clippy::pedantic`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#default_trait_access

tests/test_chain.rs

@@ -51,7 +51,7 @@ fn test_len() {
 
 #[test]
 fn test_default() {
-    let mut c: Chain = Default::default();
+    let mut c = Chain::default();
     assert!(c.next().is_none());
 }