custom: force packer install to 1.5.1

The problem with freebsd is due to packer attempting
to set ssh credentials with the temporary key before
the EC2 instance is ready.  Rather than figure out how
to get it working and risk a future release breaking
something else, let's just stick with a known working
version.

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: I97c748029d07bbac6c1f663e1a7d1dc6f52363ab
1 file changed