Update refinerycms-core.gemspec

I'm working on integrating spree + refinerycms and have a problem with incompatible jquery-rails versions.
For example:
spree use s.add_dependency 'jquery-rails', '~> 2.2.0'
refinerycms use s.add_dependency 'jquery-rails',  '~> 2.0.0'

This patch fixes the problem.