Fix build, add Rails 6 support, and update ruby versions

[p8]

Removes ruby 2.3.8 from the build matrix because it is end-of-life and fails with Rails 6.

[bluemihai]

Hi @p8 thanks for this...

I wanted to add Foundation to my Rails 6 app and was struggling (brand new to webpacker, not a front-end dev by any means).

Does your pull request fix it? If yes, is there any way for me to roll my own before it gets merged and included in the gem? (I'm also fairly new to open-source and github, so thanks for bearing with me)

Thanks!

[p8]

Hi @bluemihai,

You can reference my fork by using the following in your Gemfile:

gem 'foundation-rails', git: 'https://github.com/p8/foundation-rails.git', ref: '199beb3' # switch to sassc and fixes reveal animation

The "Rails 6 support" is for running the specs on this project. I'm running it on Rails 6 with sprockets...

[asadakbar]

Will this be merged in soon?