Travis-CI config for GStreamer dependency

[SimonSapin]

@ferjm Is it really necessary to build gstreamer from source like you do in #21158 in order to be able to build Servo on Travis-CI? Adding dependencies per README in servo/servo-with-rust-nightly@91c90e4 didn’t work: https://travis-ci.org/servo/servo-with-rust-nightly/jobs/416837548

[SimonSapin]

Ah sorry, I didn’t pay enough attention to the error message beyond "it’s pkg-config"

Requested 'gstreamer-1.0 >= 1.12' but version of GStreamer is 1.2.4

So GStreamer is found, but not a recent enough version. What would it take to support older versions?

[jdm]

The restriction about versions comes from https://github.com/sdroege/gstreamer-rs/#linuxbsds.

[ferjm]

IIUC we are not building GStreamer from source. We fetch a tarball with 1.14.1 binaries already built for
trusty. Check #21158 (comment)

[ferjm]

It seems that this was already fixed in servo/servo-with-rust-nightly@1ada617 , so I am closing this issue. Please, reopen if you still think it's valid.

[SimonSapin]

I fixed the immediate build failures by copying the config in this repo, but downloading a random tarball still doesn’t feel like a satisfying solution. Hopefully we’ll figure out something better at some point.