travis: unpin nightly Rust

Forgot to do this, uh, *checks watch*, 4 months ago.
diff --git a/.travis.yml b/.travis.yml
index 21287b3..636c574 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -46,7 +46,7 @@
       - make docker-build
    - name: "nightly Linux x86_64"
      language: rust
-     rust: nightly-2020-06-10
+     rust: nightly
      addons:
        apt:
          packages: