Update postfix attribute, constrain postfix, bump version to 1.1.2

Seems the `postfix` cookbook has renamed a bunch of its attributes.
This commit ensures we follow suit & use the right attribute names.

We also constrain postfix pessimistically to 3.0.x.

See opscode-cookbooks/postfix@3.0.0 for details on the changes
Merge branch 'refs/heads/constrain-postfix'

* refs/heads/constrain-postfix:
  Update postfix attribute, constrain postfix, bump version to 1.1.2

Conflicts:
	metadata.rb
Bump version to 1.2.1 to include postfix fixes