Clean-up "resque-retry.gemspec"

* Use semantic versioning for development dependencies to suppress
  warnings when building the gem
* Add "version.rb" per standard convention
* Add "MIT" declaration to license file and gemspec
* Add git-ignores for tag-files and locally cut gems
Merge pull request #92 from jzaleski/jwz-clean-up-gemspec

Clean-up "resque-retry.gemspec"