Report deployment target using cap's stage 

This is an improvement for two reasons:

* `rails_env` is not set yet when the starting task is invoked, so it always reports deployments to production.
* A project might (rarely) have several stages that use the same rails env, so stage is more specific.
Merge pull request #1 from humanpractice/report_stage_instead_of_rails_env

Report deployment target using cap's stage