Fix error when reverting changed migration.

When reverting changed migration, the down would fail because the foreign key constraint has to be dropped before you are able to rename the column.

database/migrations/2014_12_24_191544_changes_for_v322.php

@@ -23,13 +23,17 @@ class ChangesForV322 extends Migration
         // rename fields
         Schema::table(
             'piggy_bank_events', function (Blueprint $table) {
+            $table->dropForeign('piggy_bank_events_piggy_bank_id_foreign');
             $table->renameColumn('piggy_bank_id', 'piggybank_id');
+            $table->foreign('piggybank_id')->references('id')->on('piggybanks')->onDelete('cascade');
         }
         );
 
         Schema::table(
             'piggybank_repetitions', function (Blueprint $table) {
+            $table->dropForeign('piggy_bank_repetitions_piggy_bank_id_foreign');
             $table->renameColumn('piggy_bank_id', 'piggybank_id');
+            $table->foreign('piggybank_id')->references('id')->on('piggybanks')->onDelete('cascade');
         }
         );