Compatibility with Spree 3.4?

[nullset2]

Environment:

OS: Windows Ubuntu (Ubuntu 16.04.3 LTS)
Rails: 5.1.4
Bundler: 1.16.0
Ruby: 2.4.1p111

Trying to implement this extension with Spree 3.4.4 and I ran into dependency issues.

Bundler could not find compatible versions for gem "spree_core":
  In snapshot (Gemfile.lock):
    spree_core (= 3.4.4)

  In Gemfile:
    spree (~> 3.4.4) was resolved to 3.4.4, which depends on
      spree_core (= 3.4.4)

    spree_product_subscriptions was resolved to 3.2.0, which depends on
      spree_core (~> 3.2.0.rc2)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

Currently testing compatibility. I think updating the dependencies may be in order.

Do you guys have a specific way to contribute or test?

[nullset2]

After generating bundle exec rails g spree_product_subscriptions:install and trying to migrate:

Would you like to run the migrations now? [Y/n]
         run  bundle exec rake db:migrate from "."
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:

[nullset2]

Checkout PR #56!