ubuntu 调整进程分数，预防被杀

Starting Gerrit Code Review: WARNING: Could not adjust Gerrit's process for the kernel's out-of-memory killer.
         This may be caused by /home/gerrit/review_site/bin/gerrit.sh not being run as root.
         Consider changing the OOM score adjustment manually for Gerrit's PID=7026 with e.g.:
         echo '-1000' | sudo tee /proc/7026/oom_score_adj