Update .cirrus.yml

Author: amontoison

.cirrus.yml

@@ -5,7 +5,6 @@ task:
         image_family: freebsd-13-3
       env:
         matrix:
-          - JULIA_VERSION: 1.6
           - JULIA_VERSION: 1
   install_script: |
     URL="https://raw.githubusercontent.com/ararslan/CirrusCI.jl/master/bin/install.sh"