make clean in the install script

As per
https://github.com/HoTT/HoTT/commit/bab2b34719108dbc5da66ac40c6fe01e64f4071b#commitcomment-7695206,
this will prevent having subtlely inconsistent .vo files when you
recompile Coq.
Merge pull request #473 from JasonGross/install-make-clean

make clean in the install script
./autogen.sh