add flags to use no cache buster for image urls

sass/base/_commons.sass

@@ -14,7 +14,7 @@ html, body
 html
 	font-size: $global-font-size
 	background-color: #ddd
-	background-image: image-url("bg.png")
+	background-image: image-url("bg.png", false, false)
 	background-attachment: fixed
 
 body

sass/layout/_menu.sass

@@ -19,7 +19,7 @@ $menuBgColor : $dark-grey
 		max-width: 100%
 		width: $site-width
 		margin: auto
-		background: image-url("logo.png") no-repeat 50% 50%
+		background: image-url("logo.png", false, false) no-repeat 50% 50%
 		background-size: 20px
 		li
 			position: relative