filter out the relation_stats parameter

without this, if you use relation_stats in your configuration, this
happens:

```
CRITICAL   2014-01-16 00:20:27,655 19372  MainProcess     MainThread newrelic_plugin_agent.plugins.postgresql      poll                      L259   : Could not connect to PostgreSQL, skipping stats run: invalid connection option "relation_stats"
```
Merge pull request #216 from 3dna/fix-postgres-relation-stats

filter out the relation_stats parameter