Revert "Disable CI builds which now also hit issue 115"

This reverts commit d168e3a.

Author: dtolnay

.github/workflows/ci.yml

@@ -15,9 +15,8 @@ jobs:
         rust:
             # pinned due to https://github.com/serde-rs/serde-rs.github.io/issues/115
           - nightly-2020-05-09
-            #- beta
-            #- stable
-          - 1.44.0
+          - beta
+          - stable
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@master