Upgraded phpseclib to version 0.3.6 (SFTP ext storage)

This should improve performance of SFTP and will also fix the warnings that appeared after merging #7979 (see https://github.com/owncloud/core/pull/7979#issuecomment-39181343)

Note that the warnings seems to be harmless and it works fine, so let's only upgrade in OC 7.

Please review @bantu @icewind1991