fix flaky spec in ip_service_spec

spec/services/ip_service_spec.rb

@@ -2,6 +2,8 @@ require 'spec_helper'
 
 describe IPService do
   describe '.ip_trusted?' do
+    after(:each) { ENV['TRUSTED_NETWORKS'] = nil }
+
     subject { IPService.ip_trusted?(ip) }
 
     context 'when the ip is nil' do