compile with rust-1.37 gives warning: trait objects without an explicit `dyn` are deprecated

[stefson]

   Compiling void v1.0.2 (/tmp/rust-void)
warning: trait objects without an explicit `dyn` are deprecated
  --> src/lib.rs:67:32
   |
67 |     fn cause(&self) -> Option<&error::Error> {
   |                                ^^^^^^^^^^^^ help: use `dyn`: `dyn error::Error`
   |
   = note: #[warn(bare_trait_objects)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.21s

you might want to fix this, before it get's turned into an error in the future? :)

[KamilaBorowska]

It won't be an error ever. There are no plans to remove anything deprecated in Rust.