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

vendor/bootstrap/scss/mixins/_reset-text.scss

@@ -5,7 +5,7 @@
   font-weight: $font-weight-normal;
   line-height: $line-height-base;
   text-align: left; // Fallback for where `start` is not supported
-  text-align: start; // stylelint-disable-line declaration-block-no-duplicate-properties
+  text-align: start;
   text-decoration: none;
   text-shadow: none;
   text-transform: none;