Switch license to GNU GPLv3+.

This was performed after receiving approval from all major Dormbase
contributors in the thread with Message-ID:
    <CAKoptiw2na1R3DeBcnSUHN2JdZYCELWdPxrssL3o8ghG4hwSqw@mail.gmail.com>

The aformentioned and its replies can be viewed via the "Discuss"
conferencing system in the "dormbase" meeting on charon.mit.edu,
starting with message 0100.