Update README.md

* updated _Current Version_  to be 2.2.1, which is the latest version available on Maven Central - see http://search.maven.org/remotecontent?filepath=com/complexible/stardog/stardog-groovy/2.2.1/stardog-groovy-2.2.1.pom

* Clarified wording of statement of dependency on stardog-spring
Merge pull request #5 from delocalizer/patch-2

Update README.md