plugins/pg_*: remove empty graph_order

This attribute without a value is causing a lot of warnings like:
> [WARNING] 1 lines had errors while 35 lines were correct in data from 'config
> pg_stat_activity_[...]' on [...]
pg_table_index_size_: Add support for postgres 9.2 for index vs. data graph
Merge branch 'master' into debian
Update changelog for 0.6-1 release