Fix memory metric on *NIX

metrics/memory.sh

@@ -13,11 +13,11 @@ if is_osx; then
   }
 else
   collect () {
-    report $(free | awk '/buffers\/cache/ {
-                              printf "%.1f", 100 - $4 / ($3 + $4) * 100.0 }')
+    report $(free | awk '/Mem:/ {
+              printf "%.1f", 100 - $4 / ($3 + $4) * 100}')
   }
 fi
 
 docs () {
   echo "Percentage of used memory."
-}
+}