refactor:Make addMember an alias of add

Duplicate code existed for adding member to an organisation

Under lib/Github/Api/Organisation/Members.php the addMember and add
methods returned the exact same code and used the exact same arguments.

With that in mind I made addMember an "alias" returning $this->add
passing the same arguments through.

Author: alexanderholman

lib/Github/Api/Organization/Members.php

@@ -44,11 +44,6 @@ public function check($organization, $username)
         return $this->get('/orgs/'.rawurlencode($organization).'/public_members/'.rawurlencode($username));
     }
 
-    public function addMember($organization, $username)
-    {
-        return $this->put('/orgs/'.rawurlencode($organization).'/memberships/'.rawurlencode($username));
-    }
-
     public function publicize($organization, $username)
     {
         return $this->put('/orgs/'.rawurlencode($organization).'/public_members/'.rawurlencode($username));
@@ -67,6 +62,11 @@ public function add($organization, $username)
         return $this->put('/orgs/'.rawurlencode($organization).'/memberships/'.rawurlencode($username));
     }
 
+	public function addMember($organization, $username)
+	{
+		return $this->add($organization, $username);
+	}
+
     public function remove($organization, $username)
     {
         return $this->delete('/orgs/'.rawurlencode($organization).'/members/'.rawurlencode($username));