Set min-width on non-responsive auto columns

Had already done this on responsive variants, missed adding it to the
base variant. Without this, auto columns are not forced to the same
width.

src/generators/grid.less

@@ -49,6 +49,7 @@
     flex-grow: 1;
     flex-basis: 0;
     max-width: 100%;
+    min-width: 0;
   }
   .col-first {
     order: -1;