BF: fix chained sympy schedulers
NF: make web hook scheduler run sympy builds

See script `master/contrib/post_build_request.py` in the buildbot repo.

Run sympy binary builds with:

    post_build_request.py --category=release --project=sympy \
        --repository=sympy --host=nipy.bic.berkeley.edu

adding `--revision=sympy-0.7.3` or similar to build a particular tag or sha1
commit.