Explicitly setting the version of beberlie/assert to a stable tag.

This is to resolve the issue of installing a dev stable package in an dependency.  This will prevent users having to explicitly define a lower minimum-stability in their root composer.json file, or having to explicitly set the dev stability for beberlei/assert.

I've confirmed tests still pass with this change, but as the package version 1.2 (the current latest) for beberlei/assert is from fall 2012, if there's any recent changes you need this fix might not work.