Define enable_extension method to prevent undefined method error

I am getting this error when trying to run `rake db:test:prepare` in my application. The problem seems to be that I am using postgresql for development but sqlite3 for testing, and the method `enable_extension` is not defined in `Sqlite3Adapter` nor `AbstractAdapter`. As the schema is dumped from the dev database, the following code in `PostgresqlAdapter` (line 70)

```
def extensions(stream)
  return unless @connection.supports_extensions?
  extensions = @connection.extensions
  if extensions.any?
    stream.puts "  # These are extensions that must be enabled in order to support this database"
    extensions.each do |extension|
      stream.puts "  enable_extension #{extension.inspect}"
    end
    stream.puts
  end
end
```

will always write the line

`enable_extension "plpgsql"`

in `schema.rb`

This patch simply adds an empty definition to prevent it. Another option to fix this would be to modify the schema dumper to write `enable_extension("plpgsql") if supports_extensions?` instead. I can adapt the pull request to this if needed.

I am not sure if this is intended or I am missing something, please let me know if there is a smarter way to fix it (or no need to fix it at all!). Also I am not sure what to test here, can someone point me in the right direction? Should I also modify the changelog?