Raise exception if endpoint is wrong format

So @cpapazian and I were bitten by forgetting to include the colon and port, and when this happens the resulting error (a failure in `remote_command()`) takes a while to track down. 

We initially wrote this as just defaulting the port to 11211 if unspecified. However, this severely increases the pain of debugging connections to non-standard ports with only a very minor usability improvement for everyone else.
(some context: Ruby's keyword `or`/`and` behave differently than `||`/`&&` because they have different precedence. The generally accepted style bans them because the distinction isn't obvious and the inconsistency makes code harder to understand)