simplify test for contexts (is now by default an empty list)

repo2docker/buildpacks/r.py

@@ -239,8 +239,7 @@ class RBuildPack(PythonBuildPack):
             ),
         ]
 
-        if self.stencila_manifest_contexts:
-            if "r" in self.stencila_manifest_contexts:
+        if "r" in self.stencila_contexts:
             scripts += [
             (
                 "${NB_USER}",