Move 3.6.1 back to 3.6.1-3bionic

Was accidentally included along with the 3.6.3-1biocnic
upgrade for 3.6

Author: yuvipanda

repo2docker/buildpacks/r.py

@@ -75,7 +75,7 @@ def r_version(self):
             "3.5.3": "3.5.3-1bionic",
             "3.6": "3.6.3-1bionic",
             "3.6.0": "3.6.0-2bionic",
-            "3.6.1": "3.6.1-1bionic",
+            "3.6.1": "3.6.1-3bionic",
             "4.0": "4.0.5-1.1804.0",
             "4.0.2": "4.0.2-1.1804.0",
             "4.1": "4.1.2-1.1804.0",