Drone doesn't remove non-reusable sessions from the queue

When running simplified test case, and stopping selenium server in mean time, session isn't reusable anymore.

Such session should be removed from the queue, but instead, non-reusable sessions are stacked, which clutters console output.

Steps to reproduce:

1. $ rm ~/.drone-driver-session-store
2. start selenium server
2. run SimplerReusableRemoteWebDriverTestCase
3. stop & start selenium server
4. run test case again
5. new session is created, and warning appears: WARNING: Unable to reuse session: 1342428569722
6. stop & start selenium server
7. run test case again
8. new session is created, but two warnings appears: WARNING: Unable to reuse session: 1342429013548 and {{WARNING: Unable to reuse session: 1342428569722
}}

Which means session 1342428569722 is still not removed from queue.