remove build folder prior to building wheels

Otherwise files that have been removed will still show up in the wheel
-- but not in the source distribution -- which is super confusing and
wrong. No idea why Python doesn't check this itself.

publish.sh

@@ -1,6 +1,7 @@
 #!/bin/bash
 set -e
 
+rm -rf build/
 rm -f dist/*
 mkdir -p dist
 python setup.py sdist bdist_wheel