Omit assets in non-skinnable builds when mod.assets === false

When a module's build config includes the "assets" property set to `false`,
we should skip copying any existing assets folder for that build target.

However, if that module's meta.json includes the "skinnable" property set
to `true`, the assets directory will still be copied, as it is required by
the skinning pattern.

Fixes #87

(Until #91 is merged, Travis will fail for unrelated reasons)