Travis: use local Dovecot installation

[Slamdunk]

• Travis: move to Dovecot pure testing
• PHP: bump to ^7.0
• Travis: test against PHP 7.0, 7.1, 7.2
• Remove and ignore composer.lock: catch forward compatibility breaks early
• Autoload tests only on dev
• PHPUnit: bump to ^5.7
• Tests: fix failures to ease this PR, new PR will come to fix the tests
• Doc: update local testing guide

[ddeboer]

Great work!

There are still four failing test cases, of which two have to do with encoding issues. Do you have any idea why Ddeboer\Imap\Tests\MessageTest::testKeepUnseen and Ddeboer\Imap\Tests\MessageTest::testDelete no longer work?

[Slamdunk]

There are still four failing test cases

Yes, I did it on purpose: first fix the test environment, and only then fix the tests

of which two have to do with encoding issues

Encoding is a nightmare; we'll need a lot of test cases and dedicated tests. I'll work on them only after everything else because they deserve special attention

Do you have any idea why Ddeboer\Imap\Tests\MessageTest::testKeepUnseen and Ddeboer\Imap\Tests\MessageTest::testDelete no longer work?

• testKeepUnseen no idea; it seems dovecot always marks retrieved messages as seen
• testDelete is easy, delete only mark the mail as deleted but not deletes it, we need to call $mailbox->expunge() to actually delete them.

[Slamdunk]

@ddeboer I consider this PR complete, but I see you requested some changes through the review, without specific changes pinpointed. Are there any more changes needed, or can we merge it?

After this one, a lot PR will follow:

1. Coding-standard integration
2. imap_alerts and imap_errors integration into the exception messages
3. Deep encoding testing
4. New features requested by issues/PRs

[ddeboer]

I was referring to the failing test cases. Do you want to fix them in separate PRs?

[Slamdunk]

Yes, I already am, because other users can better track what happened and why:

1. One PR to move to Dovecot, and how
2. One PR to analyze and fix the errors