prefer R to conda

allows mixing conda and R because R inherits from Python (and thereby conda)

repo2docker/app.py

@@ -66,8 +66,8 @@ class Repo2Docker(Application):
             LegacyBinderDockerBuildPack,
             DockerBuildPack,
             JuliaBuildPack,
-            CondaBuildPack,
             RBuildPack,
+            CondaBuildPack,
             PythonBuildPack,
         ],
         config=True,