.form-control: Use $input-border-width instead of $border-width

[skip sauce]

scss/_forms.scss

@@ -14,7 +14,7 @@
   background-color: $input-bg;
   // Reset unusual Firefox-on-Android default style; see https://github.com/necolas/normalize.css/issues/214.
   background-image: none;
-  border: $border-width solid $input-border-color;
+  border: $input-border-width solid $input-border-color;
   // Note: This has no effect on <select>s in some browsers, due to the limited stylability of `<select>`s in CSS.
   @include border-radius($input-border-radius);
   @include box-shadow($input-box-shadow);