Avoid redeclaring variable

Capitalize the constructor to avoid redeclaration.
Fixes strict mode error.

lib/cluster.js

@@ -41,11 +41,13 @@ if (process.env.NODE_DEBUG && /cluster/.test(process.env.NODE_DEBUG)) {
 }
 
 // cluster object:
-function cluster() {
+function Cluster() {
   EventEmitter.call(this);
 }
-util.inherits(cluster, EventEmitter);
+
-var cluster = module.exports = new cluster();
+util.inherits(Cluster, EventEmitter);
+
+var cluster = module.exports = new Cluster();
 
 // Used in the master:
 var masterStarted = false;