issue #20, errors in migration

db/migrate/20160413234140_create_users.rb

@@ -1,6 +1,8 @@
 class CreateUsers < ActiveRecord::Migration
-    def change
-    	drop_table :users
+  def change
+    if table_exists? :users
+      drop_table :users
+    end
     create_table(:users) do |t|
       ## Database authenticatable
       t.string :userid,              null: false, default: ""