CSS: Use bootstrap variable for button roundness

src/styles/_welcome.sass

@@ -312,7 +312,7 @@ section.pricing
 				transform: scale(1)
 
 			a.sign-up-now
-				+button($color-primary, 3px, true)
+				+button($color-primary, $btn-border-radius, true)
 
 		h3
 			font-size: 1.8em
@@ -361,7 +361,7 @@ section.pricing
 			transform: translateX(-50%)
 			font-size: 1.2em
 
-			+button($color-primary, 3px)
+			+button($color-primary, $btn-border-radius)
 			padding: 5px 25px
 			white-space: nowrap
 			text-align: center