Updating topic and channel length

The NSQ docs specify that a channel and/or topic name can be up to 64 characters in length. Note: this is a change that ocurred in the NSQ 0.2.28 release hence the version bump in .travis.yml.

Version notes [on the NSQ release page](https://github.com/bitly/nsq/releases) indicate no backwards incompatible changes (with regard to upgrading from 0.2.27).

Fixes https://github.com/dudleycarr/nsqjs/issues/40