Fixes #156 by always using --abbrev=7 in git describe calls to ensure hashes are consistent.

I went with 7 characters, because that's also the length used when using git rev-parse --short HEAD when describe is not available.

This will make sure that the version strings generated by versioneer are consistent across different machines and environments.

(I recently ran into an issue with a development version of dask where the short hash varied between environments)