Use a ternary statement in logging code

See https://github.com/badges/shields/pull/1040#discussion_r136196343.

Closes #1040.

lib/log.js

@@ -2,11 +2,7 @@
 // eg. 4 becomes 04 but 17 stays 17.
 function pad(string) {
   string = String(string);
-  if (string.length < 2) {
-    return "0" + string;
-  } else {
-    return string;
-  }
+  return (string.length < 2) ? ('0' + string) : string;
 }
 
 // Compact date representation.