title: Django-storages not detecting changed static files
body: <p>I'm using django-storages and amazon s3 for my static files. Following the documentation, I put these settings in my settings.py</p>
  
  <pre><code>STATIC_URL = 'https://mybucket.s3.amazonaws.com/'
  
  ADMIN_MEDIA_PREFIX = 'https://mybucket.s3.amazonaws.com/admin/'
  
  INSTALLED_APPS += (
      'storages',
  )
  
  DEFAULT_FILE_STORAGE = 'storages.backends.s3boto.S3BotoStorage'
  AWS_ACCESS_KEY_ID = 'mybucket_key_id'
  AWS_SECRET_ACCESS_KEY = 'mybucket_access_key'
  AWS_STORAGE_BUCKET_NAME = 'mybucket'
  STATICFILES_STORAGE = 'storages.backends.s3boto.S3BotoStorage'
  </code></pre>
  
  <p>And the first time I ran collect static everything worked correctly and my static files were uploaded to my s3 bucket. </p>
  
  <p>However, after making changes to my static files and running <code>python manage.py collectstatic</code> this is outputted despite the fact that static files were modified</p>
  
  <pre><code>-----&gt; Collecting static files
      0 static files copied, 81 unmodified.
  </code></pre>
  
  <p>However, if I rename the changed static file, the changed static file is correctly copied to my s3 bucket. </p>
  
  <p>Why isn't django-storages uploading my changed static files? Is there a configuration problem or is the problem deeper?</p>