Minor but important improvements

• Better error-handling in certain relatively common cases (like specifying
  a non-existent environment)
• Print command output in real-time when running with -v (verbose), so you
  don't have to wait five minutes to see what happened.

Just a version bump

The previous upload to rubygems.org wasn't made off a tag, so it looks like
a pre-release. Let's make a "real" release this time.

Initial release of giddyup-deploy

I've had this lurking around in my tree for long enough; time to get it out
there.