Call branch proc by default in deploy.rb template.

If a proc is used in an ask and no input is entered for the ask, `question.rb` will use the default value, which is a proc object and *not* its returned value. 

Without `.call`, we can get issues like the following, from `capistrano-rsync`:

Code:

```ruby
checkout = %W[git reset --hard origin/#{fetch(:branch)}]
Kernel.system *checkout
```

Error:

```
Please enter branch: |master|

fatal: Invalid object name 'origin/#<Proc'.
cap aborted!
```