`check_for_sed_presence()` false negative on OS X

[iandunn]

check_for_sed_presence() calls sed --version to test if sed is installed on the system. That works for GNU sed in most Linux distributions, but OS X ships with FreeBSD's version, which doesn't support the --version argument.

> wp mu-migration import tables example.sql --blog_id=3 --old_prefix=wp_ --new_prefix=wp_3_ --old_url=example.net --new_url=example.localhost
Error: sed not present, please install sed

> which sed
/usr/bin/sed

> sed --version
sed: illegal option -- -
usage: sed script [-Ealn] [-i extension] [file ...]
       sed [-Ealn] [-i extension] [-e script] ... [-f script_file] ... [file ...]