added support for high sierra and updated about this mac picture to be current

Author: logeyg

sites/en/installfest/img/AboutThisMac.png

@@ -4,6 +4,7 @@ step "Learn your Mac OS X Version" do
 * Click on the Apple icon in the top left of your screen.
 * Select "About This Mac"
 * In the window that comes up, under the title "Mac OS X" there will be a version number.
+  * If it starts with 10.13, you have **High Sierra**.
   * If it starts with 10.12, you have **Sierra**.
   * If it starts with 10.11, you have **El Capitan**.
   * If it starts with 10.10, you have **Yosemite**.
@@ -32,7 +33,7 @@ end
 
 step "Choose your instructions" do
 
-  option "Sierra/El Capitan/Yosemite/Mavericks/Mountain Lion/Lion/Snow Leopard" do
+  option "High Sierra/Sierra/El Capitan/Yosemite/Mavericks/Mountain Lion/Lion/Snow Leopard" do
     link "osx_rvm"
   end
   option "Earlier than Snow Leopard" do