no trailing commas in json

gistfile1.sh

$ perl -MJSON -E 'say encode_json [1, 2, 3,]'
[1,2,3]

$ python -c 'import json; print(json.dumps([1, 2, 3,]))'
[1, 2, 3]

$ ruby -r json -e 'puts [1, 2, 3,].to_json'
[1,2,3]

$ coffee -e 'console.log [1, 2, 3,]'
[ 1, 2, 3 ]