add Imports and Depends to DESCRIPTION

[wenjie2wang]

Nice package.

This PR suggests a minor modification to the DESCRIPTION file.

I think it would be great to add Imports and Depends entries to the DESCRIPTION so that people can easily know the package is light-weighted and does not require many other packages installed from CRAN. I had to check the NAMESPACE and found out it only require utils package (and base R of course).

As for the Depends, I just checked time of the first commit for this package (Sept. 2015) and put in the latest version of R at that time. You may relax the requirement. But it would be good to have one. After all the package itself is about "versions".

Feel free to decline this PR if you find it unnecessary.