Allow deprecated Error::cause (#818)

Error::cause is deprecated in Rust 1.33, but this allows Error::cause
until the minimum supported version of tokio is Rust 1.30.

When the minimum support version of tokio reaches Rust 1.30,
replace Error::cause with Error::source.

Fixes: #817

src/runtime/current_thread/runtime.rs

@@ -92,6 +92,10 @@ impl Error for RunError {
     fn description(&self) -> &str {
         self.inner.description()
     }
+
+    // FIXME(taiki-e): When the minimum support version of tokio reaches Rust 1.30,
+    // replace this with Error::source.
+    #[allow(deprecated)]
     fn cause(&self) -> Option<&Error> {
         self.inner.cause()
     }