Add Gitter to README (JuliaLang#17878)

* Add Gitter to README

The Gitter is a large active community that new users have found lots of help at. I think it would be helpful to have the Gitter right here, because some of the people went to IRC for chat only to find out about the Gitter through the Gitter/IRC bot (and then switched to Gtiter for the code highlighting). I think showing the active community and the help it gives on a README will be important to lower the new-user threshold.

* Removed Google+, and moved Gitter above IRC

README.md

@@ -28,6 +28,7 @@ This is the GitHub repository of Julia source code, including instructions for c
 - **Source code:** <https://github.com/JuliaLang/julia>
 - **Git clone URL:** <git:https://github.com/JuliaLang/julia.git>
 - **Mailing lists:** <https://julialang.org/community/>
+- **Gitter:** <https://gitter.im/JuliaLang/julia>
 - **IRC:** <https://webchat.freenode.net/?channels=julia>
 - **Code coverage:** <https://coveralls.io/r/JuliaLang/julia>
 
@@ -42,7 +43,6 @@ developers may find the notes in [CONTRIBUTING](https://github.com/JuliaLang/jul
 - [**StackOverflow**](https://stackoverflow.com/questions/tagged/julia-lang)
 - [**Youtube**](https://www.youtube.com/channel/UC9IuUwwE2xdjQUT_LMLONoA)
 - [**Twitter**](https://twitter.com/JuliaLanguage)
-- [**Google+**](https://plus.google.com/communities/111295162646766639102)
 - [**Meetup**](https://julia.meetup.com/)
 
 <a name="Currently-Supported-Platforms"/>