Better express the dependency on spree_product_asssembly

[thisiscab]

A major problem right now is that the way we are using the assembly gem
in our fork is different of the original implementation.

Instead of a product having parts (which is the case in our fork),
variants have parts (in the original implementation). We'll have to
figure something out.

This solves #37

[thisiscab]

The problem I had expressed:

Instead of a product having parts (which is the case in our fork),
variants have parts (in the original implementation). We'll have to
figure something out.

Was not true. I just had made a dummy mistake :)

[braidn]

@cabouffard did you ping anyone here for a Code review?

[thisiscab]

Ping @braidn @DanielWright @bryanmtl
Here is part of the argument: #37