Adapting gemspec after 94945e3c2587592e4f023e1c4ceaac3bbc9e30d1

History.md had been renamed to CHANGELOG.md, when trying to install resque via bundler produces "...did not have a valid gemspec."
Merge pull request #864 from carhartl/patch-1

Adapting gemspec after 94945e3c2587592e4f023e1c4ceaac3bbc9e30d1