qa: ensure composer.lock uses code compatible with PHP 5.6, 7.0

doctrine/migrations v2 requires ocramious/package-versions and
ocramius/proxy-manager, both of which make use of PHP 7.1+ features. As
such, in order to install even when platform dependencies are ignored,
we need the base requirements to target doctrine/migrations v1.

This patch represents installing with PHP 5.6.

Author: weierophinney