Don't load spring gem in bin/spring when spring is disabled

[micahtessler]

This disables the loading of the spring gem in bin/spring when spring id disabled.

This code fix addresses a bug that occurs under the following circumstances:

• A rails project has spring installed for developers as a development depencency
• The automated tests are run in a test container that has the test dependencies, but not the development depencencies
• The tests contain this recommend block from Active Record

  # Checks for pending migrations and applies them before tests are run.
  # If you are not using ActiveRecord, you can remove these lines.
  begin
  ActiveRecord::Migration.maintain_test_schema!
  rescue ActiveRecord::PendingMigrationError => exception
  puts exception.to_s.strip
  exit 1
  end

• That code invokes this line from ActiveRecord: https://github.com/rails/rails/blob/08b3cc3d3254d92647a0812a9c9b6f8b22375ac6/activerecord/lib/active_record/migration.rb#L652 system("bin/rails db:test:prepare")
• The bin/rails call runs bin/spring
• bin/spring attempts to load the spring gem which fails

With the additional check for SPRING_DISABLED env variable, rails tests do not attempt to load spring when the env variable is set.