Bump the sandbox version

The playbook the CI job runs trips over the PPA upgrade in
https://github.com/edx/configuration/pull/3470

util/jenkins/ansible-provision.sh

@@ -127,7 +127,7 @@ fi
 
 if [[ -z $ami ]]; then
   if [[ $server_type == "full_edx_installation" ]]; then
-    ami="ami-79a18c6e"
+    ami="ami-42d9f255"
   elif [[ $server_type == "ubuntu_12.04" || $server_type == "full_edx_installation_from_scratch" ]]; then
     ami="ami-1fd6fb08"
   elif [[ $server_type == "ubuntu_14.04(experimental)" ]]; then