Overrides the zend-hydrator plugins to use the extensions provided in
this repository. This preserves the original typehints, allowing
developers to gradually migrate by replacing zend-stdlib typehints with
zend-hydrator typehints.

As noted in zendframework/zendframework#7672, on updating to:

- zend-stdlib >= 2.7.0
- zend-mvc >= 2.6.0

backwards compatibility is broken with regards to typehints against hydrators.
The reason is two-fold:

- The `HydratorPluginManager` is simply an extension of the one provided in
  zend-hydrator, meaning that all plugins returned are under the `Zend\Hydrator`
  namespace, not the `Zend\Stdlib\Hydrator` namespace.
- zend-mvc altered the `HydratorPluginManager` mapping to use the instance from
  zend-hydrator instead of zend-stdlib.

This patch addresses the first point of each. It does the following:

- Updates the DelegatingHydratorFactory to return a zend-stdlib instance.
- Updates the HydratorPluginManager to override the defaults from zend-hydrator,
  and have them return zend-stdlib extensions.
- Adds a test suite for HydratorPluginManager to verify backwards compatibility.

A related commit against the zend-mvc 2.6 series will occur shortly, having it
use the zend-stdlib HydratorPluginManager.

- Added `COMPOSER_ROOT_VERSION` directive to composer install, to ensure we
  don't get a circular dependency issue.
- PHP 7 MUST pass!