.travis.yml: add sndio to FreeBSD run.

Also add necessary environment variables and move to /latest, which has
sndio-1.7.0.

.travis.yml

@@ -29,7 +29,9 @@ jobs:
       compiler: clang
       env:
       before_install:
-        - sudo pkg install -y gtk-layer-shell gtkmm30 jsoncpp libdbusmenu
+        - export CPPFLAGS+=-isystem/usr/local/include LDFLAGS+=-L/usr/local/lib # sndio
+        - sudo sed -i '' 's/quarterly/latest/' /etc/pkg/FreeBSD.conf
+        - sudo pkg install -y gtk-layer-shell gtkmm30 jsoncpp libdbusmenu sndio
                libfmt libmpdclient libudev-devd meson pulseaudio scdoc spdlog
       script:
         - meson build -Dman-pages=enabled