Update link to ninja homepage.

(The old link forwards to the new one these days.)

Author: nico

Diff for: README.md

@@ -86,7 +86,7 @@ command line tools to your `PATH`:
 
     export PATH=/Applications/CMake.app/Contents/bin:$PATH
 
-[Ninja](http://martine.github.io/ninja/) is the current recommended build system
+[Ninja](https://ninja-build.org) is the current recommended build system
 for building Swift and is the default configuration generated by CMake. If
 you're on OS X or don't install it as part of your Linux distribution, clone
 it next to the other projects and it will be bootstrapped automatically: