(#355) Remediate SASS Depreciation Warnings

This remediates two SASS depreciation warnings that were occurring when
running `gulp`. The version of SASS choco-theme uses has been pinned to
a specific major and minor version, while still allowing patch releases.

In addition, a percentage has been passed to a SASS variable instead
of just 0, as required.

package.json

@@ -72,7 +72,7 @@
     "mousetrap": "^1.6.5",
     "nouislider": "^15.7.0",
     "rimraf": "^3.0.2",
-    "sass": "^1.53.0",
+    "sass": "~1.64.2",
     "stylelint": "^14.14.0",
     "stylelint-config-standard": "^29.0.0",
     "stylelint-config-standard-scss": "^5.0.0",

scss/variables/_boxstarter.scss

@@ -57,8 +57,8 @@ $alert-border-radius:               0;
 $alert-border-width:                0;
 $alert-margin-bottom:               0;
 $alert-border-width:                0;
-$alert-bg-scale:                    0;
-$alert-color-scale:                 0;
+$alert-bg-scale:                    0%;
+$alert-color-scale:                 0%;
 
 // Tabs
 $nav-tabs-link-active-bg:           $white;

scss/variables/_chocolatey.scss

@@ -45,8 +45,8 @@ $alert-border-radius: 0;
 $alert-border-width: 0;
 $alert-margin-bottom: 0;
 $alert-border-width: 0;
-$alert-bg-scale: 0;
-$alert-color-scale: 0;
+$alert-bg-scale: 0%;
+$alert-color-scale: 0%;
 
 // Tabs
 $nav-tabs-link-active-bg: $white;