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

scripts/setup-ubuntu.sh

@@ -58,10 +58,13 @@ sudo_warn
 # Setup all apt packages we need
 #
 echo_info -x 'Preparing and downloading packages we need...'
-PKGS="build-essential pkg-config sqlite3 tzdata"
-#PKGS="$PKGS libavcodec-dev libavformat-dev libavutil-dev"
-PKGS="$PKGS libncurses5-dev libncursesw5-dev"
-PKGS="$PKGS libsqlite3-dev libssl-dev"
+PKGS="build-essential \
+      libncurses5-dev \
+      libncursesw5-dev \
+      libsqlite3-dev \
+      libssl-dev \
+      pkgconf \
+      tzdata"
 
 # Add yarn before NodeSource so it can all go in one update
 #
@@ -93,7 +96,7 @@ fi
 #
 if [ ${DO_UPDATE:-1} ]; then sudo apt-get update -y; fi
 
-# Install necessary pakackes
+# Install necessary packages
 #
 sudo apt-get install -y $PKGS
 sudo apt-get autoremove -y