Update phpseclib from 0.3.6 to 0.3.7.

@DeepDiver1975 @PVince81 

Refs
https://github.com/owncloud/core/issues/9320#issuecomment-48567982
https://github.com/owncloud/core/pull/9198
https://github.com/phpseclib/phpseclib/issues/391

Needs backport to stable7