gemspec: Made it ignore the private key, we don/'t build/publish it as a...

Hi,
I wanted to install rye from your github via an entry in my Gemfile like this:

gem "rye", :github => 'delano/rye', :ref => 'abcdef123'

But: bundler install complained about your missing private key.

So I modified your gemspec that it will not try to sign, if it is installed directly.

rgds,
j