Update R 3.4 from 3.4.0 to 3.4.4

repo2docker/buildpacks/r.py

@@ -70,7 +70,7 @@ class RBuildPack(PythonBuildPack):
             "4.0": "4.0.5",
             "3.6": "3.6.3",
             "3.5": "3.5.3",
-            "3.4": "3.4.0",
+            "3.4": "3.4.4",
             "3.3": "3.3.3",
         }