perl-regex not compatible on MacOS

[natalieda]

The get-macaroon option to specify the duration of a macaroon does not work on MacOS because grep won't recognise the option `--perl-regex':

mac:dcache_api_tests anatolid$ ./get-macaroon --url https://dolphin12.grid.surfsara.nl:2880/users/anatolid/ --duration PT5M --chroot --user anatolid --permissions DOWNLOAD,UPLOAD,MANAGE,LIST --output rclone tokenfile
grep: unrecognized option `--perl-regex'
usage: grep [-abcDEFGHhIiJLlmnOoqRSsUVvwxZ] [-A num] [-B num] [-C[num]]
	[-e pattern] [-f file] [--binary-files=value] [--color=when]
	[--context[=num]] [--directories=action] [--label] [--line-buffered]
	[--null] [pattern] [file ...]
Duration should be in ISO8601 duration format. Examples: PT5M (5 minutes), P1Y2M (1 year and 2 months)

[onnozweers]

Fixed in 963139b.