Fix theories/Functorish.v

I'm not sure whether or not the last line in that proof is needed in an
old version of HoTT/coq, but I'm pretty sure it's not needed in the
newest version.  Chaining it with the previous command and adding a
[try] permits it to be run only if necessary.
Merge pull request #164 from JasonGross/fix-functorish

Fix theories/Functorish.v