v5.1.3: Merge pull request #1717 from haiwen/deb-qt5

Revert "Require qt 5.5+ in deb packages."

v5.1.2

[client] Update version to 5.1.2.

v5.1.3-server

Updated scripts for windows server 5.1.

v5.1.2-server

Remove redundant debug log messages.

v5.1.1

Don't compile with syslog support for client and Windows.

v5.1.1-server

Use Qt5 on linux.

v5.0.7

[client] Update version to 5.0.7.

v5.1.0-server

modify seahub.sql for 5.1 db upgrade

v5.0.6

[client] Remove -Werror flag from makefile.

v5.0.5-server: Always delete built-in ignored files when apply delete-dir operation …

…in download.