Ensures that Rails options are split by whitespace when generating Rails 6.y.z apps

[jrgriffiniii]

I apologize if this is not the proper approach to addressing this problem, please close this if it is not fit to be merged.

I was attempting to pass --skip-javascript within the .engine_cart.yml file in order to test a Gem against Rails 6.0.0 releases, and found that this was not being whitespace-separated with other Rails generator arguments. Also, I found that I needed to introduce several changes to ensure that the latest Travis CI builds pass for Rails 6.0.0 with sprockets 4.0.0 releases.