Implement SimpleCov coverage reporting.
Allow 'rs_tag --list' to fail gracefully.

If the prefetch of tags fails, the worst case is that Puppet
re-tags the host with its own tags -- which is fine. This allows
the execution of 'rs_tag --list' to fail gracefully, and the
module still works.
Disable the final coverage report for now.

It only works in Ruby 1.9+, and I'm not sure how to make
it gracefully exit in Ruby 1.8.7 yet.
Completely disable the SimpleCov module until it works in 1.8
Merge pull request #1 from Nextdoor/fix_rstag_failure_handling

Fix rstag failure handling