Use describe on feature tests

The `type: :feature` is automatically detected by RSpec because these
tests are inside the `spec/features` folder. Using `feature` re-adds a
`type: :feature` to these files, which will result in a conflict when we
upgrade to Rails 5.1's system tests.

Because of this change, we also need to change `background` to `before`
or else these tests will fail.

spec/features/admin/administrators_spec.rb

@@ -1,11 +1,11 @@
 require "rails_helper"
 
-feature "Admin administrators" do
+describe "Admin administrators" do
   let!(:admin) { create(:administrator) }
   let!(:user) { create(:user, username: "Jose Luis Balbin") }
   let!(:user_administrator) { create(:administrator) }
 
-  background do
+  before do
     login_as(admin.user)
     visit admin_administrators_path
   end
@@ -58,7 +58,7 @@ feature "Admin administrators" do
                                                                  username: "Tony Soprano",
                                                                  email: "tony@soprano.com")) }
 
-    background do
+    before do
       visit admin_administrators_path
     end