Load rake task under test for all its specs

If you ran only the second spec without the first, the rake file wasn't
loaded and the test failed.

spec/lib/tasks/users_rake_spec.rb

@@ -4,12 +4,14 @@ require 'spec_helper'
 require 'rake'
 
 describe 'users.rake' do
+  before(:all) do
+    Rake.application.rake_require 'tasks/users'
+    Rake::Task.define_task(:environment)
+  end
+
   describe ':remove_enterprise_limit' do
     context 'when the user exists' do
       it 'sets the enterprise_limit to the maximum integer' do
-        Rake.application.rake_require 'tasks/users'
-        Rake::Task.define_task(:environment)
-
         max_integer = 2_147_483_647
         user = create(:user)