Upgrading to Shrine 3.x

[akhsunna]

Considering that derivatives plugin replaced versions plugin in shrine 3.x, some code should be updated.

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Also, I added link_name option to RailsAdmin::Config::Fields::Types::FileUpload. I hope it's okay. I did it because currently links for shrine fields don't look nice:

[prem-prakash]

tested here, did not work

when there is already an attached file, the attachment data is not loaded on a hidden field, and if the user do not upload a new file, it gives an error:

Shrine::InvalidFile ("" is not a valid IO object (it doesn't respond to #read, #eof?, #rewind, #close)):

[prem-prakash]

I was able to make it work.
It seems that the option cache: true is necessary for the plugin model

  plugin :model, cache: true # false won't work

[akhsunna]

So I fixed the cache_method and tests for ruby 2.2.

Travis CI: I'm not sure why the tests randomly fail on the "nested form" part :(
Code Climate: this is how it was before, so probably we can ignore these issues within this pull request

[prem-prakash]

shouldn't there be an else with the versions plugin?

[akhsunna]

thumb method will work only with derivatives
https://github.com/sferik/rails_admin/blob/e4d970e96a845f20ed898ebd6456062bb13a422c/lib/rails_admin/config/fields/types/shrine.rb#L10-L13

It won't work with version of shrine older than v3. So I just skipped tests about image versions:
https://github.com/sferik/rails_admin/blob/e4d970e96a845f20ed898ebd6456062bb13a422c/spec/rails_admin/config/fields/types/shrine_spec.rb#L14-L20

@prem-prakash, do you think we need to support thumbs for older versions of shrine too?

[prem-prakash]

@akhsunna I would say that it would be a good practice at least to not break with older versions, what do you think?

[akhsunna]

Okay, maybe it's worth it :) Will add it soon

[mshibuya]

Merged in, since this is necessary in attempt to run Ruby 3.0 CI build.
Thanks for the contribution!