Set nginx dependency to ~> 2.4.4

Nginx cookbook 2.5.0 has broken the way this cookbook's nginx_load_balancer is expecting to generate templates.  Commit: https://github.com/opscode-cookbooks/nginx/commit/094685b2c649d80fb7c58b0cf0805820dcbe174c

The proper fix is probably to adjust nginx_load_balancer, but this quick dependency changes solves the problem for me and may be useful to others until a full patch is made.