[WIP] Add MariaDB addon

## DO NOT MERGE YET

This PR adds MariaDB addon, based on instructions given at https://downloads.mariadb.org/mariadb/repositories/#mirror=digitalocean-nyc&distro=Ubuntu&distro_release=precise&version=10.0.

Versions 5.5, 10.0, and 10.1 can be specified in `.travis.yml`; e.g.,

```yaml
addons:
  mariadb: "10.0"
```

Note that MariaDB is installed with the package-provided configuration file `-o Dpkg::Options::='--force-confnew'`, so that it is run on disk, not ramfs, so it can be potentially slow.

User `travis` needs to be set up, but for some reason `root` cannot.

```sh-session
travis@debug-BanzaiMan-playing-around-php-1412545209:~$ mysql -u root 
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 252
Server version: 10.0.14-MariaDB-1~precise-log mariadb.org binary distribution

Copyright (c) 2000, 2014, Oracle, SkySQL Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> CREATE USER 'travis'@'%' IDENTIFIED BY '';          GRANT ALL PRIVILEGES ON *.* TO 'travis'@'%';
ERROR 1396 (HY000): Operation CREATE USER failed for 'travis'@'%'
Query OK, 0 rows affected (0.00 sec)
```