Updated to version 0.15.1

This release fixes a serious security issue found in the way that RSA keys
were being generated.

It recommended that existing Salt keys be regenerated once 0.15.1 has been
deployed on the master and all minions.
A 'key_regen' routine has been added to 0.15.1 to make this transition easier.
The following sequence is a convenient way to regenerate all keys in an
environment:

salt-run manage.key_regen

You will be prompted to restart the master. Once completed, all keys in the
environment will have been regenerated and you will need to accept the new
keys using the following command:

salt-key -A

(imil)