add gem version badge to README

@bess - Is it possible you tagged a new version but didn't make a new gem release?  I'm going through my email of last week;  found a message about a travis build passing for ?? a tag  0.0.24 ??:

https://travis-ci.org/sul-dlss/mods/builds/30028693

Do we need to cut a new gem release?  I can show you how.

@lmcglohon  -- can you do a fast-forward merge for this (maybe show bess how to merge a pull request with a fast-forward merge) and then I can show you both how to release the gem?

It will need:  updated version number in the lib/version file;  I also try to put comments at the bottom of the readme ...

AND, see #6 -- we should add coveralls for code coverage to this too.

Isn't maintenance fun????