Improve CI setup for parallelism

[ArturT]

Hi, I noticed you run on CircleCI tests in 4 parallel containers. I work on knapsack_pro ruby gem for running tests across parallel CI containers in optimal time and I was wondering if this would be helpful for your CI setup. I added recently free support for open source projects like yours and I was wondering if Spree could benefit from it.

To give you a working project example, see Consul which is an open-source project. They use Knapsack Pro and here is Consul CI build recorded with knapsack_pro ruby gem and tests are split across parallel CI jobs thanks to Knapsack Pro Queue Mode. All parallel CI nodes finished work at a similar time so thanks to that there is no bottleneck and the CI build time can be as fast as possible.

What do you think?