Avoid path constraints to packer binary

[mcandre]

Please drop the GOPATH-relative path to packer and just runpacker instead. That way, the packer-freebsd configuration integrates better with multiple ways that packer can be installed, such as via brew install packer. Currently, packer-freebsd requires that packer be installed specifically by go get github.com/hashicorp/packer, which is unnecessarily restrictive.