longer hash part in version string

thanks @petrrr for the notification.

some links don't resolve with short hases, e.g.:

https://github.com/obspy/obspy/tree/0.9.2-787-gf176
https://github.com/obspy/obspy/tree/0.9.2-787-gf176f4c

versus:

https://github.com/obspy/obspy/tree/0.9.2-787-gf176f4cd
https://github.com/obspy/obspy/tree/0.9.2-787-gf176f4cdae