* upgrade min required rust to 1.21; crossbeam-deque requires the
ord_max_min feature, apparently stabilized in this version.
* use "make --jobs=2" to build ffmpeg so it goes faster.
https://docs.travis-ci.com/user/reference/overview/ says there are 2
cores available.
* upgrade minimum required Rust from 1.17 to 1.20; reffers 0.4.2
apparently uses std::mem::ManuallyDrop, introduced in 1.20
* install ffmpeg from source (requiring sudo access) rather than using
the ancient one from Ubuntu Trusty to meet the minimum version
requirements specified in ffmpeg/build.rs.
Scott Lamb