Revert "Pin Travis to nightly-2019-10-01"

The bug has been fixed as of nightly-2019-10-03.

This reverts commit 2eb77e6.

Author: dtolnay

.travis.yml

@@ -2,9 +2,7 @@ language: rust
 
 matrix:
   include:
-      # Pinned due to a bug in nightly:
-      # https://github.com/rust-lang/rust/issues/64964
-    - rust: nightly-2019-10-01
+    - rust: nightly
       script: cargo test
       env: RUSTFLAGS='--cfg async_trait_nightly_testing'
     - rust: stable