Update GitHub download URLs in README

From `raw.github.com' to `raw.githubusercontent.com'

Fixes #4933

README.markdown

@@ -26,13 +26,13 @@ Oh My Zsh is installed by running one of the following commands in your terminal
 #### via curl
 
 ```shell
-sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
+sh -c "$(curl -fsSL https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
 ```
 
 #### via wget
 
 ```shell
-sh -c "$(wget https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh -O -)"
+sh -c "$(wget https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh -O -)"
 ```
 
 ## Using Oh My Zsh