Bug fix for issue #24 (regression introduced in 0.11)

When I released pip-accel 0.11 some logic was changed (handling of
requirements with a `satisfied_by' value). This introduced a fairly
subtle bug which I never saw until an issue on GitHub and a complaint
by a colleague pointed out the problem. Here's the issue on GitHub:

  https://github.com/paylogic/pip-accel/issues/24