update bootstrap 4.3.1, update gulpfile, npm start script added, change SCSS file names for proper compiling

vendor/bootstrap/scss/mixins/_image.scss

@@ -20,7 +20,6 @@
 //
 // Short retina mixin for setting background-image and -size.
 
-// stylelint-disable indentation, media-query-list-comma-newline-after
 @mixin img-retina($file-1x, $file-2x, $width-1x, $height-1x) {
   background-image: url($file-1x);
 
@@ -29,8 +28,9 @@
   // There's no such thing as unprefixed min-device-pixel-ratio since it's nonstandard.
   // Compatibility info: https://caniuse.com/#feat=css-media-resolution
   @media only screen and (min-resolution: 192dpi), // IE9-11 don't support dppx
-  only screen and (min-resolution: 2dppx) { // Standardized
+    only screen and (min-resolution: 2dppx) { // Standardized
     background-image: url($file-2x);
     background-size: $width-1x $height-1x;
   }
+  @include deprecate("`img-retina()`", "v4.3.0", "v5");
 }