test(donation_context): fix any_instance deprecation warning

Changed from the old syntax to the new allow_any_instance_of according to the documentation.

https://www.rubydoc.info/github/rspec/rspec-mocks/RSpec/Mocks/ExampleMethods%3aallow_any_instance_of

spec/support/contexts/shared_rd_donation_value_context.rb

@@ -19,7 +19,7 @@ RSpec.shared_context :shared_rd_donation_value_context do
 
   let(:default_edit_token) {'7903e34c-10fe-11e8-9ead-d302c690bee4'}
   before(:each){
-    Event.any_instance.stub(:geocode).and_return([1,1])
+    allow_any_instance_of(Event).to_receive(:geocode).and_return([1,1])
   }
 
 
@@ -495,4 +495,4 @@ RSpec.shared_context :shared_rd_donation_value_context do
   def nil_or_true(item)
     item.nil? || item
   end
-end
+end