Use a better model for tune=ssim
Use a better model for tune=ssim

Comparing to the baseline tune=ssim, the average gains are
PSNR -0.55, SSIM -0.30, MS-SSIM -0.98, VMAF -1.26

Details: (150f VBR)
               PSNR    SSIM  MS-SSIM    VMAF
Lowres       -1.347   0.291   -0.307  -1.291
Midres       -0.628  -0.329   -1.011  -2.173
Hdres         0.781  -0.656   -1.319   0.210
Ugc360p      -2.695  -0.972   -1.503  -4.055
Lowres_bd10   0.074   0.196   -0.623  -0.835
Midres_bd10   0.517  -0.327   -1.124   0.566

Change-Id: Ie034eaedf20e1fe843921cafbb3b7ad9a2bc89d1

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx