travis: bump up osx_image version to 10.2
travis: bump up osx_image version to 10.2

To avoid re-compiling homebrew dependencies.

Cinegy/srt,Haivision/srt,ethouris/srt,Haivision/srt,ethouris/srt,ethouris/srt,Cinegy/srt,Haivision/srt,Cinegy/srt,Cinegy/srt,ethouris/srt,Haivision/srt