patch 8.2.0417: Travis CI config can be improved

Problem:    Travis CI config can be improved.
Solution:   Remove COVERAGE variable. Add load-snd-dummy script. add "-i NONE"
            to avoid messages about viminfo. (Ozaki Kiichi, closes #5813)

ci/load-snd-dummy.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -e
+
+if ! modprobe snd-dummy; then
+    # snd-dummy is contained in linux-modules-extra (if exists)
+    apt install -y "linux-modules-extra-$(uname -r)"
+    modprobe snd-dummy
+fi