Add support for rails 6.0

[hackartisan]

And drop support for rails versions less than 5.2 and ruby 2.4.

closes #184

[jrgriffiniii]

This seems to have been blocked by a prior release of hydra-editor:

Bundler could not find compatible versions for gem "sprockets":
  In snapshot (Gemfile.lock):
    sprockets (= 4.0.2)

  In Gemfile:
    hydra-editor was resolved to 5.0.3, which depends on
      sprockets (>= 3.7)

I can look to address this and diagnose the update conflicts.

[jrgriffiniii]

I am going to retain the branch for this, however, I feel that it is best to proceed by closing this PR in order to start the process of cleaning this project.