Don’t look issues on inactive projects

The rake task for checking for inactive projects was allowing any project with an open issue, even if it's not had a commit in 6 months.

In a future update we should switch to using @kdaigle's score checker from https://github.com/24pullrequests/24pullrequests/pull/705 and run this task on a more regular basis.

Fixes https://github.com/24pullrequests/24pullrequests/issues/697