Use https when possible.

scss/_custom-forms.scss

@@ -157,7 +157,7 @@
 // Select
 //
 // Replaces the browser default select with a custom one, mostly pulled from
-// http://primercss.io.
+// https://primer.github.io/.
 //
 
 .custom-select {

scss/_print.scss

@@ -5,7 +5,7 @@
 // ==========================================================================
 // Print styles.
 // Inlined to avoid the additional HTTP request:
-// http://www.phpied.com/delay-loading-your-print-css/
+// https://www.phpied.com/delay-loading-your-print-css/
 // ==========================================================================
 
 @if $enable-print-styles {

scss/mixins/_screen-reader.scss

@@ -1,6 +1,6 @@
 // Only display content to screen readers
 //
-// See: http://a11yproject.com/posts/how-to-hide-content/
+// See: https://a11yproject.com/posts/how-to-hide-content/
 // See: https://hugogiraudel.com/2016/10/13/css-hide-and-seek/
 
 @mixin sr-only {