ActiveRecord's use of RAILS_ROOT for sqlite drop db operation

Quick change from RAILS_ROOT to Merb.root so that dropping databases
now works with sqlite.

Signed-off-by: Michael S. Klishin <michael@novemberain.com>

merb_activerecord/lib/active_record/merbtasks.rb

@@ -57,7 +57,7 @@ def drop_database(config)
     when 'mysql'
       ActiveRecord::Base.connection.drop_database config[:database]
     when /^sqlite/
-      FileUtils.rm(File.join(RAILS_ROOT, config[:database]))
+      FileUtils.rm(File.join(Merb.root, config[:database]))
     when 'postgresql'
       ActiveRecord::Base.clear_active_connections!    
       `dropdb "#{config[:database]}"`