Avoid an unnecessary allocation in recvMessage

Creating a readBuf here is a bit of an overkill since it's only used to
read the length of the message.  Invoke encoding/binary directly to
avoid the allocation, resulting in a ~10% performance gain in a
benchmark as described in #270.
Test against Go 1.3 in Travis

Go 1.3 was released on 2014-06-18.
Fix copypaste-o in Listener docs