add capistrano ~> 2.15 to gemspec as a dependency

pd-cap-recipes without capistrano doesn't make sense. thus, we are
adding it to gemspe.

2.15 because we would like to start using smarter asset compilation and
rollback features that were released in recent releases of capistrano.

https://github.com/capistrano/capistrano/pull/281
bump version to 0.0.7
Merge pull request #24 from PagerDuty/capistrano-in-gemspec

capistrano in gemspec