fixes lint warnings using single quoted on icons.scss

app/assets/stylesheets/icons.scss

@@ -270,21 +270,21 @@
 }
 
 .icon-calculator::before {
-  content: "\36";
+  content: '\36';
 }
 
 .icon-map-marker::before {
-  content: "\37";
+  content: '\37';
 }
 
 .icon-user-plus::before {
-  content: "\38";
+  content: '\38';
 }
 
 .icon-file-text-o::before {
-  content: "\39";
+  content: '\39';
 }
 
 .icon-file-text::before {
-  content: "\21";
+  content: '\21';
 }