try upgrading travis-ci setup to xenial

This no longer requires installing ffmpeg manually, so there should be
significantly less data to cache (faster runs). The build step itself
should also be faster when the cache is unavailable/stale.

Also sneak in a change from "pkg-config" to "pkgconf" package in the
scripts and travis CI. They didn't match the manual instructions; make
them all consistent. They both seem to work fine, but I gather pkgconf
is the newer thing. Its roadmap is here and notes that distros are
moving toward it.

https://github.com/pkgconf/pkgconf/wiki/Roadmap

ci/install-ffmpeg.sh

@@ -1,13 +0,0 @@
-#!/bin/bash -e
-if [ ! -f ffmpeg-${FFMPEG_VERSION}/configure ]; then
-  wget https://ffmpeg.org/releases/ffmpeg-${FFMPEG_VERSION}.tar.xz
-  tar xf ffmpeg-${FFMPEG_VERSION}.tar.xz
-fi
-cd ffmpeg-${FFMPEG_VERSION}
-./configure --enable-shared
-make --jobs=2
-sudo make install --jobs=2
-sudo ldconfig
-
-# The build log varies with each invocation; remove it to improve cachability.
-rm -f ffbuild/config.log