Enable setting blacklight-version

[jcoyne]

We do this in the github config, but we never use that value

Fixes #3025

[jcoyne]

Failures are possibly due to one of these problems:

    generate  spotlight:install
       rails  generate spotlight:install -f --mailer_default_url_host=localhost:3000
warning: parser/current is loading parser/ruby32, which recognizes 3.2.4-compliant syntax, but you are running 3.2.3.
Please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
The file /Users/jcoyne85/workspace/projectblacklight/spotlight/.internal_test_app/app/assets/javascripts/application.js does not appear to exist
      append  app/assets/config/manifest.js
       force  app/controllers/catalog_controller.rb
    rakefile  spotlight_test.rake
      create  config/initializers/carrierwave.rb
      create  app/assets/images/spotlight/themes/default_preview.png
      create  app/assets/images/spotlight/themes/modern_preview.png
      create  app/assets/stylesheets/application_modern.css
      append  config/initializers/assets.rb
The file /Users/jcoyne85/workspace/projectblacklight/spotlight/.internal_test_app/config/initializers/spotlight_initializer.rb does not appear to exist