Add Go playground (#2531)

* Add Go playground

* Fix Travis build

free-programming-playgrounds.md

@@ -4,6 +4,8 @@
 * [Crystal](#crystal)
 * [CSS](#css)
 * [FlexBox](#flexbox)
+* [Go](#go)
+* [Haskell](#haskell)
 * [JavaScript](#javascript)
 * [.Net](#dotnet)
 * [Node.js](#nodejs)
@@ -38,6 +40,11 @@
 * [Echoplex](http://the-echoplex.net/flexyboxes/)
 
 
+### Go
+
+* [Go Playground](https://play.golang.org)
+
+
 ### Haskell
 
 * [Try Haskell](https://www.tryhaskell.org)