Fix Redis service to use the latest version and to work on OS X

Hi,

I hope this is a useful contribution. 
To make the Redis service work on my Mac OS X 10.9.1 environment, I made the following changes:

* Updated reference to Redis v 2.8.6.
* Changed downloadPath to point to the new URL for Redis.
* Placed a check to avoid recompiling Redis.
* Added a chmod task to make mkreleasehdr.sh executable for _make_ to work on Mac OS X 10.9.1 with Cloudify 2.7.0 GA (build 5996). 

This should work on other Unix platforms. Further testing required on discrete platforms.

Kind regards,
Fuzz.