update dogapi dependency, refs #28

This is pretty safe, since most recent installs will even have a
newer version of dogapi installed, as 1.6 was released February 19, 2013,
so any nodes launched after that will be unaffected.
update logic for tag-setting to be Chef-specific only

- simplified `get_combined_tags`
  - no longer needs hostname, as it only needs Chef::Node object now
  - pass along the 'chef' source type to enable `tag:sometag` in Datadog
- renamed `host_tags` to `chef_env`, since it no longer looks up current host tags
- dropped `get_host_tags` methods
- updated test suite to ensure that no `GET` calls for tags are made
- re-recorded VCR cassettes for added `source` parameter

Fixes #28
Merge pull request #43 from DataDog/28_update_tags

update logic for tag-setting to be Chef-specific only