[build] Travis: Build with ninja instead of make on Linux (#981)
[build] Travis: Build with ninja instead of make on Linux (#981)

ninja automatically makes use of all available hardware threads, and
should be considerably faster than `make -j$(nproc)` as well.

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