Use sass/webpack references

Via: https://github.com/angular/angular-cli/issues/2115\#issuecomment-247244983

Author: peterblazejewicz

src/styles.scss

@@ -1,5 +1,5 @@
 /* You can add global styles to this file, and also import other style files */
-@import "../node_modules/bootstrap/scss/bootstrap";
+@import "~bootstrap/scss/bootstrap";
 
 body {
   padding-top: 5rem;