Update XCrySDen definition file

Updated XCrySDen definition file to bootstrap from the new base
Ubuntu 18.04 container that is part of the new naked-singularity
container collection on Singularity Hub.

definition-files/xcrysden/Singularity.xcrysden-1.6.2

 Bootstrap: shub
-From: mkandes/ubuntu:latest
+From: mkandes/naked-singularity:ubuntu-18.04
 
 %labels
 
@@ -13,7 +13,7 @@ From: mkandes/ubuntu:latest
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200413
+    LAST_UPDATED 20201231
 
 %setup