[Hackday] Update Heroku doc with SYMFONY_ENV param

| Q             | A
| ------------- | ---
| Doc fix?      | [yes]
| New docs?     | [no]
| Applies to    | [2.5, 2.6, 2.7]
| Fixed tickets |  #4217
That is true, @mxnr, but as @timglabisch pointed out, it really should be done this way on all deploys, not just on deploys to Heroku. Otherwise, all the `post-install-cmd` scripts run in the `dev` env. @fabpot agreed that this is the right way across the board.