Dont set default publisher when community site is false

@fnichol I noticed that publisher was still being called even when community site publishing was disabled.  This PR is to avoid that.

I played with a few options in releaser.rb.  The first was:

``` ruby
@publisher = options.fetch(:publisher) { default_publisher } if publish_to_community
```

but it was too long and tailor let me know it! :smile: 

Also tried an if statement in the initializer:

``` ruby
if publish_to_community
  @publisher = options.fetch(:publisher) { default_publisher }
end
```

but it just didn't feel as clean since everything else was a one liner.  I can change it back though if that's what you prefer.