added $MongoDB::VERSION so that MakeMaker wouldn't complain (VERSION was...

... present in 7.000.0, but not in the current tree)

The CPAN repository is still showing 7.000.0 as the most current version so I went looking for the version mentioned on the mongodb website because I was running into the javascript test failures that are reportedly fixed in 7.000.1 and 7.001.0

I do know know why or when this package version line was removed, but restoring it, made MakeMaker happy and compiled and passed all the tests on OpenSuSE and Mac OS X 10.8