migrating partition to a 2015 MacBook Pro
You can use Thunerbolt/FireWire to boot a Mac into Target-Mode to use it as a drive. Than you can clone it using disk utility, like you would clone a regular external drive. Next you'll (probably) need to reinstall OS X on the new MacBook Pro over that partition (so you have the latest OS X with all the drivers for the new device and whatnot), you can either do that by first creating a bootable USB (DiskMaker X is an easy solution for that) or by using Internet Recovery (boot the Mac while holding cmd+alt/option+R).