Updated slack link in README.md

Updated the Slack link to reference the Julia Language Slack page.

Closes #542

README.md

@@ -7,7 +7,7 @@
 [![Docs](https://img.shields.io/badge/docs-stable-blue.svg)](https://JuliaPOMDP.github.io/POMDPs.jl/stable)
 [![Dev-Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://JuliaPOMDP.github.io/POMDPs.jl/latest)
 [![Gitter](https://badges.gitter.im/JuliaPOMDP/Lobby.svg)](https://gitter.im/JuliaPOMDP/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
-[![Slack](https://img.shields.io/badge/Chat%20on%20Slack-with%20%23pomdp--bridged-ff69b4)](https://slackinvite.julialang.org)
+[![Slack](https://img.shields.io/badge/Chat%20on%20Slack-with%20%23pomdp--bridged-ff69b4)](https://julialang.org/slack/)
 
 This package provides a core interface for working with [Markov decision processes (MDPs)](https://en.wikipedia.org/wiki/Markov_decision_process) and [partially observable Markov decision processes (POMDPs)](https://en.wikipedia.org/wiki/Partially_observable_Markov_decision_process).
 The [POMDPTools](https://juliapomdp.github.io/POMDPs.jl/stable/POMDPTools/#pomdptools_section) package acts as a "standard library" for the POMDPs.jl interface, providing implementations of commonly-used components such as policies, belief updaters, distributions, and simulators.