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.
In general, the Ruby `or` keyword is very poorly behaved so we should probably avoid it. https://github.com/bbatsov/ruby-style-guide#no-and-or-or

How about something like
```ruby
if (match = ENDPOINT_REGEX.match(endpoint))
  @host = match[1]
  @port = match[2].to_i
else
  raise ArgumentError.new("Couldn't parse hostname and port from endpoint: #{endpoint}")
end
```