title: Rails + Postgis: error cannot drop table spatial_ref_sys
body: <p>Rails 5<br>
  Postgresql 9.4.10<br>
  Postgis 2.1.8<br></p>
  
  <p>when I try rake db:reset, console show an error</p>
  
  <blockquote>
  <pre><code>rake aborted! ActiveRecord::StatementInvalid:
  PG::DependentObjectsStillExist: ERROR:  cannot drop table
  spatial_ref_sys because extension postgis requires it HINT:  You can
  drop extension postgis instead. : DROP TABLE "spatial_ref_sys" CASCADE
  </code></pre>
  </blockquote>
  
  <p>I am new to psql and postgis, any help will be appreciated.</p>