chore/Update default electricity unit rate [CP-1074]

Default unit rate for the energy appliance calculator will change to 27.69 pence per kWh from around 1st January.
See  jira ticket.

Author: KPobeeNorris

config/application.rb

@@ -77,6 +77,6 @@ class Application < Rails::Application
 
     config.session_store :disabled
 
-    config.x.default_unit_rate = 26.35
+    config.x.default_unit_rate = 27.69
   end
 end