Update Travis config.
Update Travis config.

- Cache for faster builds.
- Skip Ruby 2.0.0 and add 2.2.x

spree-contrib/spree-product-assembly,helloluis/spree-product-assembly,tmedford/spree-product-assembly,helloluis/spree-product-assembly,tmedford/spree-product-assembly,glossier/spree-product-assembly,spree-contrib/spree-product-assembly,helloluis/spree-product-assembly,tmedford/spree-product-assembly,spree-contrib/spree-product-assembly,BeDepraved/spree-product-assembly,glossier/spree-product-assembly,glossier/spree-product-assembly,BeDepraved/spree-product-assembly,BeDepraved/spree-product-assembly