Add support to replace php.ini settings

From https://github.com/laravel/homestead/pull/87

>Making it possible to replace php.ini settings through Homestead.yml like this:

>```
php_ini_overrides:
    - max_execution_time: "300"
    - memory_limit: 2048M
    - post_max_size: 100M
    - upload_max_filesize: 100M
```