Error when running 'gulp production'

[jamint]

Hey Jian. Love the project!!!

When running gulp.production I'm getting the following error:

events.js:160
throw er; // Unhandled 'error' event
^
CssSyntaxError: /Users/jad.mintun/Desktop/visit london/visitlondon-master/src/scss/_grid.scss:30:28: Unknown word
You tried to parse SCSS with the standard CSS parser; try again with the postcss-scss parser 28 | @include tablet {
29 | @for $i from 1 through $columns {

30 | .medium-column-#{$i} {
| ^
31 | width: 100% / $columns * $i;
32 | }