Override predefined variables

ie. so changing values in `.env` actually works

The problems we're trying to solve with dotenv:
 * we want settings from environment variables to come in to our app [PASS] 
 * we're using unicorn and restarting (with zero downtime), but we need our
   environment variables to be override-able in each new fork  [FAIL - the values
   are inherited from their parernt]

Which is why we need to change the current behaviour of dotenv to override predefined variables.
(Perhaps there is a way of configuring the gem to have either behaviour?)