Replace DIRECTORY_SEPARATOR with '/' (#1) (#69)

This pull request aims to fix zips created on windows with a broken folder structure which doesn't match the lambda function handler.

src/Package.php

@@ -413,7 +413,7 @@ protected function prependBasePath($path)
  {
  $path = $this->removeBasePath($path);
 
- return $this->getBasePath() . ($path ? DIRECTORY_SEPARATOR . $path : $path);
+ return $this->getBasePath() . ($path ? '/' . $path : $path);
  }
 
  /**