Remove flex shorthand

sass/components/nav.sass

@@ -68,7 +68,8 @@ a.nav-item
 .nav-left
   align-items: stretch
   display: flex
-  flex: 1
+  flex-basis: 0
+  flex-grow: 1
   justify-content: flex-start
   overflow: hidden
   overflow-x: auto
@@ -78,13 +79,16 @@ a.nav-item
   align-items: stretch
   display: flex
   justify-content: center
+  margin-left: auto
+  margin-right: auto
 
 .nav-right
   // Responsiveness
   +tablet
     align-items: stretch
     display: flex
-    flex: 1
+    flex-basis: 0
+    flex-grow: 1
     justify-content: flex-end
 
 // Main container