configurable deployment annotation names

I wanted to distinguish deployment annotations between our staging and production environments so I made the annotation configurable with an environment variable called 'LIBRATO_DEPLOYMENT_ANNOTATION'. The annotation name still defaults to "deployments" if the env var isn't present. Let me know what you think.
Ahh.  Good call. I added a `to_sym` call in and rebased the branch down to one commit.  https://github.com/spacechurro/librato-rake-deploytrack/commit/cc2aa6d0e042becf27d4a43cda41ade3756137bd