Fix dropUnique() errors in migration file.

This can't be the best solution, but works for me.
I'm on MySQL version 5.5.38.

Adding the following to migrations produces errors:
public function up()
    {
        Schema::table('federation_clients', function ($table) {
            $table->dropUnique('key');
        });
    }

error when running the migration:
SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'key' at line 1 (SQL: alter table `federation_clients` drop index key)

The unique key is actually: federation_clients_key_unique

Table creation:
Schema::create('federation_clients', function ($table) {
            $table->increments('id');
            $table->string('key')->unique();
        });
    }