Add compatibility for the Symfony 3 directory structure

I've been using this in two new Symfony projects for a couple of months now, one of them is quite large and I haven't had any issues. However, my Ruby skills aren't the best and given this project supports Symfony 1 and Symfony 2, there should probably be a `capifony_symfony3.rb` file also.

So to use it with the new directory structure, you just configure:

```ruby
set :use_symfony_3_dirs, true
```

However in the composer.json for a new Symfony project, there are the options `symfony-var-dir` and `symfony-bin-dir`. If these are customized, then you configure these to match also.

```ruby
set :bin_path, "bin"
set :var_path, "var"
```