Install Windows 7 On MacBook (x) 2015
You can install unsupported operating systems such as Linux or older Windows versions by not using the bootcamp assistant. 

Open disk utility and create a partition, format it FAT. 
Then reboot whole holding alt/option and boot from the install-disc (CD, DVD, USB drive, ...) and install as usual. 

Now comes the tricky part. Since the OS is not officially supported by Apple your on your own regarding driver-install. You'll be able to just use the regular drivers from the vendors for things like audio and wifi, for apple specific drivers, you can use the drivers that are downloadable by boot camp assistant. They single drivers are usually easily accessible in the package, you just can't use the Install-Everything App that Apple bundles with them. 

Alternatively, use virtualization is a good option.