Add responses for when the bot is mentioned

2 jobs for master in 3 seconds (queued for 5 seconds)
Name Stage Failure
test Test
   3c86300..6435434  master     -> origin/master
Checking out 64354344 as master...
Skipping Git submodules setup
Checking cache for default...
time="2020-04-18T23:09:23+08:00" level=error msg="Docker executor: prebuilt image helpers will be loaded from /var/lib/gitlab-runner."
Successfully extracted cache
$ rustc --version
bash: line 78: rustc: command not found
ERROR: Job failed: exit status 1