Change the default key frame interval from 30 to 240 (#769)

Change the default key frame interval from 30 to 240

For the test set vc-360p-1
master_gop_size_30@2018-11-26T21:44:33.537Z -> master_gop_size_240@2018-11-26T19:54:21.900Z

    PSNR |  PSNR Cb |  PSNR Cr | PSNR HVS |     SSIM |  MS SSIM | CIEDE 2000
-24.0682 | -17.4493 | -17.5778 | -24.1882 | -24.5156 | -24.3186 |   -21.7343