Remove special handling for 'postgis' schema

Before merging this, I would like your feedback. Motivation: I don't think this adapter should encourage installing PostGIS into a non-standard schema.

To test this locally:

```
gem 'activerecord-postgis-adapter', github: 'rgeo/activerecord-postgis-adapter', branch: 'postgis_schema'
```

By default, the PostGIS extension installs PostGIS tables into the 'public' schema. There is no reason for this adapter to do anything special for a 'postgis' schema. PostGIS tables are excluded from the structure dump via ignore_tables, not via the schema.

The default postgres #structure_dump method handles multiple schemas via 'schema_search_path' in database.yml. If you want install PostGIS tables into a non-default schema, add that schema to your 'schema_search_path' like so:

database.yml:
```
schema_search_path: 'public, postgis'
```

https://github.com/rails/docrails/blob/master/activerecord/lib/active_record/tasks/postgresql_database_tasks.rb

http://lists.osgeo.org/pipermail/postgis-users/2009-November/025165.html

Related: #73 and #97. This PR would make those obsolete.