Add complete support for HTTP-based test methods on DrupalWebTestCase 

I think very few people use the unit test components of SimpleTest in the Drupal ecosystem.  Seems like the Travis config should also, at a minimum, spin up a server so that methods like DrupalWebTestCase::drupalGet() and  DrupalWebTestCase::drupalPost() are functional.

Drush provides this capability (but it requires an extra PHP package to do so in PHP 5.3).  We also need PDO MySQL, since we're working with a MySQL database.

Naturally, others may want to install Apache with all requirements, or nginx, or try against sqllite / postgres, but this provides a good outline of the bare minimum.