Update the stable version to SRS 4.0 (#2802)
Update the stable version to SRS 4.0 (#2802)

now 4.0 is released and stable.
VERSION_STABLE should be 4
ossrs/srs,winlinvip/srs,winlinvip/srs,ossrs/srs,ossrs/srs,winlinvip/srs,ossrs/srs,ossrs/srs,ossrs/srs,winlinvip/srs,winlinvip/simple-rtmp-server,winlinvip/srs,winlinvip/srs