Use constraints for "unique" on Postgres schemas.

Previous was using CREATE INDEX, should be using ADD CONSTRAINT as this will create the index automatically per Postgres documentation.
Signed-off-by: Taylor Otwell <taylorotwell@gmail.com>

laravel/database/schema/grammars/postgres.php

@@ -146,7 +146,11 @@ class Postgres extends Grammar {
 	 */
 	public function unique(Table $table, Fluent $command)
 	{
-		return $this->key($table, $command, true);
+		$table = $this->wrap($table);
+
+		$columns = $this->columnize($command->columns);
+
+		return "ALTER TABLE $table ADD CONSTRAINT ".$command->name." UNIQUE ($columns)";
 	}
 
 	/**
@@ -283,7 +287,7 @@ class Postgres extends Grammar {
 	 */
 	public function drop_unique(Table $table, Fluent $command)
 	{
-		return $this->drop_key($table, $command);
+		return "ALTER TABLE ".$this->wrap($table)." DROP CONSTRAINT ".$command->name;
 	}
 
 	/**