use recommended way of getting the version number

pkg_resources.require() should not be called directly, it raises
DistributionNotFound() for tornado and nose in Debian (and probably
others).  get_distribution() gives you the version as well and is not
recommended against in the documentation, so use it.
Merge pull request #5 from Vicho/master

use pkg_resources.get_distribution instead of pkg_resources.require for the version number