Bump Kino version in explore notebooks (#744)

lib/livebook/notebook/explore/distributed_portals_with_elixir.livemd

@@ -200,7 +200,7 @@ install the [Kino](https://github.com/livebook-dev/kino) library:
 
 ```elixir
 Mix.install([
-  {:kino, "~> 0.3.1", github: "livebook-dev/kino"}
+  {:kino, "~> 0.4.0"}
 ])
 ```
 

lib/livebook/notebook/explore/elixir_and_livebook.livemd

@@ -51,7 +51,7 @@ instance, otherwise the command below will fail.
 
 ```elixir
 Mix.install([
-  {:kino, "~> 0.3.1", github: "livebook-dev/kino"}
+  {:kino, "~> 0.4.0"}
 ])
 ```
 

lib/livebook/notebook/explore/intro_to_kino.livemd

@@ -10,7 +10,7 @@ and interact with them.
 
 ```elixir
 Mix.install([
-  {:kino, "~> 0.3.1"},
+  {:kino, "~> 0.4.0"},
   {:vega_lite, "~> 0.1.2"}
 ])
 ```

lib/livebook/notebook/explore/intro_to_vega_lite.livemd

@@ -11,7 +11,7 @@ directly, but it is required to render VegaLite:
 ```elixir
 Mix.install([
   {:vega_lite, "~> 0.1.2"},
-  {:kino, "~> 0.3.1"}
+  {:kino, "~> 0.4.0"}
 ])
 ```
 

lib/livebook/notebook/explore/pong.livemd

@@ -17,7 +17,7 @@ the visualization and interactions.
 
 ```elixir
 Mix.install([
-  {:kino, "~> 0.3.1", github: "livebook-dev/kino"}
+  {:kino, "~> 0.4.0"}
 ])
 ```
 

lib/livebook/notebook/explore/vm_introspection.livemd

@@ -14,7 +14,7 @@ so let's add `:vega_lite` and `:kino` for that.
 ```elixir
 Mix.install([
   {:vega_lite, "~> 0.1.2"},
-  {:kino, "~> 0.3.1", github: "livebook-dev/kino"}
+  {:kino, "~> 0.4.0"}
 ])
 ```