Update .cirrus.yml

Author: amontoison

.cirrus.yml

@@ -1,12 +1,10 @@
 freebsd_instance:
-  image: freebsd-12-1-release-amd64
+  image: freebsd-13-3-release-amd64
 task:
   name: FreeBSD
   env:
     matrix:
-      - JULIA_VERSION: 1.6
       - JULIA_VERSION: 1
-      - JULIA_VERSION: nightly
   allow_failures: $JULIA_VERSION == 'nightly'
   install_script:
     - sh -c "$(fetch https://raw.githubusercontent.com/ararslan/CirrusCI.jl/master/bin/install.sh -o -)"