Change bootstrap to source version

bootstrap/scss/mixins/_image.scss

@@ -0,0 +1,16 @@
+// Image Mixins
+// - Responsive image
+// - Retina image
+
+
+// Responsive image
+//
+// Keep images from scaling beyond the width of their parents.
+
+@mixin img-fluid {
+  // Part 1: Set a maximum relative to the parent
+  max-width: 100%;
+  // Part 2: Override the height to auto, otherwise images will be stretched
+  // when setting a width and height attribute on the img element.
+  height: auto;
+}