CI: use 1.60 as the MSRV

Author: tarcieri

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.41.0
+          - 1.60.0
     steps:
       - name: checkout
         uses: actions/checkout@v2