Issue with get_modified_time for SFTP Storage

[tompap]

Hello, starting from 1.14
"Breaking: The long deprecated & removed (from Django) (modified|created|accessed)time methods have been removed from the various storages, please replace with the get(modified|created|accessed)_time methods"

The modified_time was removed but get_modified_time was not added for SFTP Storage.

so we cannot get modified_time :/

[tompap]

PR: #1347