bump old nightly to one with ptr_cast

Author: RalfJung

.travis.yml

@@ -12,7 +12,7 @@ matrix:
     - env: TARGET=x86_64-unknown-linux-gnu
       # Don't remove this target; test coverage in `smoke.rs` relies on us running at least
       # one pinned toolchain.
-      rust: nightly-2019-06-01
+      rust: nightly-2019-08-01
       if: branch = staging OR branch = trying
 
     # Linux (build only)