fix: Deprecation warning (#72)

## Description
Updates an if statement to resolve the following deprecation warning:

```DEPRECATION WARNING [if-function]: The Sass if() syntax is deprecated in favor of the modern CSS syntax.```

Author: randombitsUK

lib/tools/_rem.scss

@@ -41,9 +41,8 @@ $gel-tools-rem-enable--fallback: true !default;
   }
 
   @return if(
-    meta.type-of($value) == number and math.unit($value) == px,
-    math.div($value, $baseline) * 1rem,
-    $value
+    sass(meta.type-of($value) == number and math.unit($value) == px): math.div($value, $baseline) *
+      1rem; else: $value
   );
 }
 

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "gel-sass-tools",
-  "version": "4.1.1",
+  "version": "4.1.2",
   "description": "A collection of Sass Settings & Tools which align to key GEL values",
   "main": "_sass-tools.scss",
   "scripts": {