Revert "Pin nightly build to 2020-05-09"

This reverts commit 40f70d8.

Author: dtolnay

.github/workflows/ci.yml

@@ -12,11 +12,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust:
-            # pinned due to https://github.com/serde-rs/serde-rs.github.io/issues/115
-          - nightly-2020-05-09
-          - beta
-          - stable
+        rust: [nightly, beta, stable]
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master