Fix Mac OS X issue in fetch-configlet script
Fix Mac OS X issue in fetch-configlet script

As @jtigger kindly pointed out in https://github.com/exercism/x-api/issues/59
the fetch-configlet script doesn't compile properly with the default bash that
ships on macs.

This copies his fix from the Java track:
https://github.com/exercism/xjava/commit/a5c990064aa66e45e9896efed5801ba704d79339

MattH-be/ecmascript,arthurchipdean/xecmascript,tejasbubane/xecmascript,MattH-be/ecmascript,arthurchipdean/xecmascript,tejasbubane/xecmascript,exercism/xecmascript,exercism/xecmascript