Update Crabgrass media to Rails 5

crabgrass-media works well with Rails 5, we just need the following changes:

• s.add_dependency 'activesupport', '>= 3.2'

• s.add_dependency 'activesupport', '~> 5.0'

• s.add_development_dependency 'minitest', '~>4.7'

• s.add_development_dependency 'minitest'

And we have to bump the version in lib/media/version.rb

Additionally, I had to make some changes to make byebug work.

I do not have write access to the Git repository. How do we publish the gem at https://rubygems.org/gems/crabgrass_media ?

I remember that we talked about replacing crabgrass_media. Just checked for the most popular libraries and found https://rubygems.org/gems/mini_magick Are there other candidates?