travis: Switch to ubuntu keyserver to avoid timeouts

.travis/lint_06_script.sh

@@ -20,6 +20,6 @@ test/lint/lint-all.sh
 
 if [ "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_EVENT_TYPE" = "cron" ]; then
     git log --merges --before="2 days ago" -1 --format='%H' > ./contrib/verify-commits/trusted-sha512-root-commit
-    while read -r LINE; do travis_retry gpg --keyserver hkp://subset.pool.sks-keyservers.net --recv-keys $LINE; done < contrib/verify-commits/trusted-keys &&
+    travis_retry gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys $(<contrib/verify-commits/trusted-keys) &&
     ./contrib/verify-commits/verify-commits.py --clean-merge=2;
 fi