force load railtie in test env

Test suite failed because railtie wasn't loaded (because
Rails/ActiveRecord are stubbed out in test env). This initializes
it explicitly in test_helper before any models get defined

test/test_helper.rb

@@ -67,6 +67,9 @@ end
 require 'geocoder'
 require "geocoder/lookups/base"
 
+# and initialize Railtie manually (since Rails::Railtie doesn't exist)
+Geocoder::Railtie.insert
+
 ##
 # Mock HTTP request to geocoding service.
 #