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.
s/Record Elimination/Nonrecursive Elimination/

As per https://coq.inria.fr/bugs/show_bug.cgi?id=3586, [Set Record
Elimination Schemes] is going away eventually.
Merge pull request #473 from JasonGross/install-make-clean

make clean in the install script
Merge pull request #474 from JasonGross/set-nonrecursive-elim

s/Set Record Elimination Schemes/Set Nonrecursive Elimination Schemes/