Add Gemfile, have gemspec specify runtime/development dependencies.

This helps someone who wants to hack on encrypted_cookie get started quicker by having all dependencies codified and installable with `bundle install`.

Along the way I specified a runtime dependency on the gem for `rack >=1.0` and changed use of `should_not raise_error` in a spec to silence an rspec deprecation warning.

I regenerated the gemspec using `bundle exec rake build_gemspec` to add the necessary dependency information. I think things are pretty much the same, I changed the `date` attribute back to what it was. The signing info was lost, though, but after looking at echoe a little I think the gem could still be signed using the previous paths by running something like:

```bash
$ GEM_PRIVATE_KEY=/home/cvk/.gemcert/gem-private_key.pem \
    GEM_CERTIFICATE_CHAIN=/home/cvk/.gemcert/gem-public_cert.pem \
    bundle exec rake package
```

Might be worth considering dropping use of echoe at some point, looks like it's no longer maintained.