added some configuration options

* smtp_encryption config parameter for main email config

http://swiftmailer.org/docs/sending.html#encrypted-smtp

* prefer_dist for composer plugin

disabling it helped me to solve the following problem:

```bash
git@host:~$ /phpci/composer.phar --prefer-dist --working-dir="/phpci/build/" install

Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
  - Installing sensio/framework-extra-bundle (2.2.x-dev 09041c1)
    Downloading: connection...
Could not fetch https://api.github.com/repos/sensio/SensioFrameworkExtraBundle/zipball/09041c1f95f4ad9c9a23eb9d23d9d110cde94a44, enter your GitHub credentials to access private repos
The credentials will be swapped for an OAuth token stored in /home/git/.composer/config.json, your password will not be stored
```

* rules option (array with rule names) for phpmd plugin