Add dependency on Ruby 2.0.0
Add dependency on Ruby 2.0.0

I'm about to import the PubSubHub files from the causes.git repo, and
they effectively depend on Ruby 2.0.0 because they use 2.0.0 syntax.

Make that dependency explicit in the gemspec.

Change-Id: I9fb4711004b402eb8e5e1150a28e0382a410daad
Reviewed-on: https://gerrit.causes.com/26272
Reviewed-by: Lann Martin <564fa7a717c51b612962ea128f146981a0e99d90@causes.com>
Tested-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>

brigade/pubsubhub