Update CUDA-aware Ubuntu + OpenMPI

Updated CUDA-aware Ubuntu + OpenMPI definition file to utilize the
Ubuntu + CUDA container from naked-singularity collection on
Singularity Hub as its base container from which to bootstrap itself.

definition-files/ubuntu/Singularity.ubuntu-18.04-cuda-10.1.168-openmpi-3.1.4

 Bootstrap: shub
-From: mkandes/ubuntu-cuda:ubuntu-18.04-cuda-10.1.168
+From: mkandes/naked-singularity:ubuntu-18.04-cuda-10.1.168
 
 %labels
 
@@ -10,7 +10,7 @@ From: mkandes/ubuntu-cuda:ubuntu-18.04-cuda-10.1.168
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200715
+    LAST_UPDATED 20201227
 
 %setup