enqueue fails if underlying connection has been reconnected

I've seen this a few times in the wild when using Amazon RDS:

```
PG::InvalidSqlStatementName: ERROR: prepared statement "que_insert_job" does not exist
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/base.rb:68:in `exec_prepared`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/base.rb:68:in `block in execute_prepared`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/active_record.rb:5:in `block in checkout`
[GEM_ROOT]/gems/activerecord-4.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in `with_connection`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/active_record.rb:30:in `checkout_activerecord_adapter`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/active_record.rb:5:in `checkout`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/base.rb:60:in `execute_prepared`
[GEM_ROOT]/gems/que-0.7.0/lib/que/adapters/base.rb:43:in `execute`
[GEM_ROOT]/gems/que-0.7.0/lib/que.rb:44:in `execute`
[GEM_ROOT]/gems/que-0.7.0/lib/que/job.rb:62:in `enqueue`
```

This might also solve the problem I've been seeing sporadically where the queue workers stop working.  I have three unicorns, and sometimes none of them do any work on the queue.