Fixed namespace issue

Summary:
PhabricatorAPIException inherits from \RuntimeException,
not from Nasqueron\Notifications\Phabricator\RuntimeException.

This issue has been noticed running phpDox:

[phpdox] [31.12.2015 - 23:48:38] -
Nasqueron\Notifications\Phabricator\PhabricatorAPIException
(missing Nasqueron\Notifications\Phabricator\RuntimeException)

Test Plan: trivial

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D144