Gerrit: accept search queries starting with '-'

Use the `--` argument to prevent gerrit from trying to parse query terms
beginning with `-` as command line flags.

Example (shows all reviews that are open and have a change in the last
hour):

hubot gerrit query status:open -age:1h
Merge pull request #1532 from docwhat/gerrit-cli-args

Gerrit: accept search queries starting with '-'