Disable CI builds which now also hit issue 115

Author: dtolnay

.github/workflows/ci.yml

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