remove pre-generated versioneer.py, build in setup.py instead

Normally ("setup.py build") we only create versioneer.py in memory, then
embed it into bin/versioneer-installer. If you want a standalone
copy (for testing or debugging), you can use the new "setup.py
make_versioneer" command, which will dump a copy in the current
directory.

This also updates the .travis.yml file to make_versioneer before running
test/test_git.py, since that test assumes it can find a copy of
versioneer.py in the top directory.