Bump minimum Node version required to 6 (#81)

Node 4 will no longer be in maintenance LTS status as of April 2018, and we are not currently running tests on Node 4, so we cannot guarantee it will work.

Author: hramos

docs/getting-started.md

@@ -214,7 +214,7 @@ brew install node
 brew install watchman
 ```
 
-If you have already installed Node on your system, make sure it is version 4 or newer.
+If you have already installed Node on your system, make sure it is version 6 or newer.
 
 [Watchman](https://facebook.github.io/watchman) is a tool by Facebook for watching changes in the filesystem. It is highly recommended you install it for better performance.
 
@@ -238,7 +238,7 @@ Open an Administrator Command Prompt (right click Command Prompt and select "Run
 choco install -y nodejs.install python2 jdk8
 ```
 
-If you have already installed Node on your system, make sure it is version 4 or newer. If you already have a JDK on your system, make sure it is version 8 or newer.
+If you have already installed Node on your system, make sure it is version 6 or newer. If you already have a JDK on your system, make sure it is version 8 or newer.
 
 > You can find additional installation options on [Node's Downloads page](https://nodejs.org/en/download/).