Composer require cdb ^1.0|^2.0 and fix unit tests for phpunit 6

[wgevaert]

When installing smw via composer on mw1.39, I get the following:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - onoi/tesa 0.1.0 requires wikimedia/cdb ~1.0 -> found wikimedia/cdb[1.0.0, ..., 1.4.1] but it conflicts with your root composer.json require (2.0.0).
    - mediawiki/semantic-media-wiki[3.2.0, ..., 3.2.3] require onoi/tesa ~0.1 -> satisfiable by onoi/tesa[0.1.0].
    - Root composer.json requires mediawiki/semantic-media-wiki ^3.2 -> satisfiable by mediawiki/semantic-media-wiki[3.2.0, 3.2.1, 3.2.2, 3.2.3].

This patch fixes that by also allowing cdb version ^2.0 (Not much has changed for the cdb versions.