use PGBOUNCER_PREPARED_STATEMENTS env var for all connections without prefix
use PGBOUNCER_PREPARED_STATEMENTS env var for all connections without prefix

in order for pgbouncer to work correctly we need to disable prepared statements.
this patch ensures the `PGBOUNCER_PREPARED_STATEMENTS` env var is used.

prior to this patch, the logs database would require a `LOGS_PGBOUNCER_PREPARED_STATEMENTS` var.
travis-ci/travis-config