Auto merge of #267 - servo:jdm-patch-6, r=jdm

Update MSRV.

once_cell 1.15.0 updated their MSRV, so we need to as well.

Author: bors-servo

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
 
     strategy:
       matrix:
-        rust: [1.49.0, nightly, beta, stable]
+        rust: [1.56.0, nightly, beta, stable]
 
     steps:
       - uses: actions/checkout@v2