Bump version to 1.6.0.
Bump version to 1.6.0.

darkside/lita-hipchat,jimmycuadra/lita-hipchat,litaio/lita-hipchat,crimsonknave/lita-hipchat,onewheelskyward/lita-hipchat,lazyfrosch/lita-xmpp,sadfuzzy/lita-xmpp