Remove backticks from the keep-alive commands

Users were confused by the backticks and using them when adding URLs for Hubot to keep alive. I don't think they add much value or make the commands any clearer, so I've removed them.
Tighten up the replies for the keep-alive script

Simple formatting changes like capitalizations, question marks and removing additional lines from the output.
Merge pull request #1100 from jhubert/update-keep-alive-docs

Small updates to the keep alive script