Change heroku deploy to master

The current heroku button doesn't work due to rauchg#164, rauchg#165 and rauchg#167

The current "doesn't work with free slacks" bug is still alive on 0.8.3 this lets idiot users like me use master until there's a tag for a branch that works.

Author: SteveC

Readme.md

@@ -21,7 +21,7 @@ Read more about the [motivations and history](http://rauchg.com/slackin) behind
 
 #### Heroku
 
-[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/rauchg/slackin/tree/0.8.3)
+[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/rauchg/slackin/tree/master)
 
 #### Azure