Fix update script on macOS

[variadico]

There is no error, but the test dir does not get updated. None of the file contents get updated.

$ sed -i "" "s/NATS_SERVER [0-9]\+\.[0-9]\+\.[0-9]\+/NATS_SERVER "2.1.4"/g" tests/build-images.sh 
$ git status
On branch master
Your branch is up to date with 'origin/master'.

[Ivan Kozlovic]

Fixed by #30