There are references in Foreman's codebase to the constant Fixnum, which is deprecated in Ruby 2.4. It is likely that replacing these with Integer is sufficient, as Fixnum is derived from Integer.

app/services/authorizer.rb
184:    @base_ids ||= @base_collection.all? { |i| i.is_a?(Fixnum) } ? @base_collection : @base_collection.map(&:id)

app/controllers/api/v2/interfaces_controller.rb
41:        param :subnet_id, Fixnum, :desc => N_("Foreman subnet ID of IPv4 interface")
42:        param :subnet6_id, Fixnum, :desc => N_("Foreman subnet ID of IPv6 interface")
43:        param :domain_id, Fixnum, :desc => N_("Foreman domain ID of interface. Required for primary interfaces on managed hosts.")

app/controllers/api/v2/external_usergroups_controller.rb
30:          param :auth_source_id, Fixnum, :required => true, :desc => N_('ID of linked authentication source')

app/models/config_report.rb
39:          when Integer, Fixnum

lib/foreman/cast.rb
11:      when Fixnum