Removed redundancy while setting vars

The README had most of the steps right, but it assumed that you set
the variables "group" and "version" in your build.gradle.  I've set
it up now so that this isn't necessary; you can just use VERSION_NAME
and GROUP without the middle man.

- Use "VERSION_NAME" instead of "version" for checking snapshot status

- Explicitly set groupId/version from GROUP and VERSION_NAME instead
  of assuming that group/version has been set previously.