Fixes #501: Default Drupal VM to PHP 5.6.

[geerlingguy]

See #501

[danepowell]

This broke VM initialization, because the package php-bz2 doesn't exist for Trusty (14.04). See #521

[geerlingguy]

@danepowell - #503 fixes this, but is waiting in a conga line of selenium/phantom-related PRs.

We could hotfix right now by setting bz2 as a php_pecl_extension instead (to make sure the 8.x HEAD branch still builds the VM until/unless selenium replaces Phantom), but we would also need to remove the bcmath installed_extra (because it also doesn't actually exist for PHP 5.6 :).

cc @grasmash — basically, we need to either get #503 merged (after phantom is no longer a requirement), or we can make a separate PR to get things building in the meantime.