Fix the vendoring script on MacOS X

The version of sed on MacOS X is different then the one on linux. The mac version
requires a parameter for the inline (-i) flag, where this isn't required on linux.
On the mac it thinks the -e flag is the parameter, and it causes the vendoring script
to fail.

This fix adds an empty string '' as a parameter to sed, which works fine on both the
mac and linux versions.

Signed-off-by: Ken Cochrane <kencochrane@gmail.com>