Fix FieldTypeHelpExtension deprecated warnings in Symfony 2.3

This pull request fixes the following warning messages:

```
DEPRECATION - getDefaultOptions() is deprecated since version 2.1 and will be removed in 2.3. Use setDefaultOptions() instead.
DEPRECATION - getAttribute() is deprecated since version 2.1 and will be removed in 2.3. Use getConfig() and FormConfigInterface::getAttribute() instead.
DEPRECATION - set() is deprecated since version 2.1 and will be removed in 2.3. Access the public property 'vars' instead.
```