Don't dump PG system views

Postgres' convention for system views is to start the view name with a
`pg_` prefix. Adding these view names to the list of ignored views when we
dump the schema to prevent conflicts with PG and extensions' views of database
metadata.

This change is meant to resolve #55.
I'm not sure we want to modify this method, given that `ignored?` will be replaced (and indeed, ignored) in near-future versions of Rails.