travis: use dl-cdn.alpinelinux.org as apk mirror

nl.alpinelinux.org is no longer the most fresh mirror. dl-cdn should be
relatively fresh while being fast

.travis/common.sh

@@ -2,7 +2,7 @@
 
 readonly ALPINE_ROOT='/mnt/alpine'
 readonly CLONE_DIR="${CLONE_DIR:-$(pwd)}"
-readonly MIRROR_URI='http://nl.alpinelinux.org/alpine/edge'
+readonly MIRROR_URI='http://dl-cdn.alpinelinux.org/alpine/edge'
 
 # Runs commands inside the Alpine chroot.
 alpine_run() {