Install giorgiosironi/eris as require-dev

Install giorgiosironi/eris as require-dev

Eris is a porting of QuickCheck and property-based testing tools to
the PHP and PHPUnit ecosystem.
It is used in WikibaseLexeme to generate inputs for tests cases.
These tests are skipped if the library is not present.
T188354 / Iba3da2820262ca8a879780ee7848cfc831df28b5 removed the
extension-specific phpunit run from composer scripts.
Due to the unavailability of eris in core composer, and the fact that
the composer merge plugin does not install require-dev of extensions,
all eris-based tests have been skipped since (2018-02-17).

Change-Id: I3909fb34025d9a9e07ccd6c85b30c34144714d04