Add ARM64 CI to Travis

We need to install a number of additional packages that are installed
by default on the AMD64 workers.

We also have to manually set up the MySQL user.

For now we don't set up Postgres -- if anyone wants to figure that
out, it would be great ;)

Log redirections in compile.sh are removed, because /dev/stdout is
not accessible. We don't see to use this anyway.

travis/setup-mysql.sh

@@ -1,2 +1,6 @@
 #!/bin/bash
+if [ -n "$ARM64" ]; then
+  sudo mysql -e 'CREATE USER "travis"@"localhost" IDENTIFIED BY ""'
+  sudo mysql -e 'GRANT ALL PRIVILEGES ON *.* TO "travis"@"localhost"'
+fi
 mysql -e "CREATE DATABASE IF NOT EXISTS test"