Make it clearer that config should not end with comma

If one simply edits the default settings, the last non-commented item finishes with a comma which causes the error:

```
ValueError: Error parsing JSON in file '/Users/tom/tmp/wcsaxes-benchmarks/asv.conf.json': Expecting property name enclosed in double quotes: line 1 column 131 (char 130)
```

It would be nice to have a more explicit error message.