Log missing directories at info level

This should fix spotify#160 "Logging is too chatty at WARN level"

src/main/java/com/spotify/fmt/AbstractFMT.java

@@ -219,7 +219,7 @@ private void handleMissingDirectory(String directoryDisplayName, File directory)
       throw new MojoFailureException(message);
     } else {
       getLog()
-          .warn(directoryDisplayName + " directory '" + directory + "' does not exist, ignoring.");
+          .info(directoryDisplayName + " directory '" + directory + "' does not exist, ignoring.");
     }
   }