Update psycopg2 to 2.5.2.

In 2.5.1 they had an issue, where OperationalError exception was causing SEGFAULT
when being pickled. This was crashing the Celery worker, causing the jobs to be lost.
Merge pull request #283 from EverythingMe/fix_stuck_jobs

Update psycopg2 to 2.5.2.