Build for multiple ruby & rails versions

[gburgett]

Now that we are using the gem in multiple apps, we should have CI build it for multiple ruby versions & rails combinations. At the very least we should build it for the combinations of our apps that use contentful:

• WM Resources
  • ruby '2.3.6'
  • gem 'rails', '~> 5.1.5'
• The Porch
  • ruby '~> 2.3.0'
  • gem 'rails', '~> 5.0.6'
• JTJ.com
  • ruby '2.3.3'
  • gem 'rails', '~> 5.0.6'
• Paper Signs (wm.org v3)
  • ruby '2.5.3'
  • gem 'rails', '~> 5.2.1'

Here's an option that Travis CI provides: https://docs.travis-ci.com/user/build-matrix/

[gburgett]

TravisCI does that now