Add testing array driver

[clementmas]

Add support for new MAILBOX_DRIVER in-memory "array" driver. It's now the default config.

Deprecate unnecessary MAILBOX_DRIVER "log" driver.
If I'm not mistaking, Mailbox doesn't log any emails. Only the Laravel MAIL_MAILER does.

So I believe most people would use these testing environment variables (inside phpunit.xml):

<server name="MAIL_MAILER" value="array"/>
<server name="MAILBOX_DRIVER" value="array"/>

Or if they want to log the test emails:

<server name="MAIL_MAILER" value="log"/>
<server name="MAILBOX_DRIVER" value="array"/>

Fixes issue #72