clarifies normalize.css is a dependency

Perhaps I'm a little slow & need to have things spelled out :)
https://github.com/twbs/bootstrap/issues/17301#issuecomment-134469886

scss/_reboot.scss

 // Reboot
 //
 // Global resets to common HTML elements and more for easier usage by Bootstrap.
-// Builds on the resets of Normalize.css.
+// Adds additional rules on top of Normalize.css.
 
 
 // Reset the box-sizing