Formula for coq v8.3pl5.

Formula is for Coq v8.3pl5, the latest version of the 8.3 branch.
This is not intended to be merged in the master branch of homebrew because it's not the latest version of Coq.
I guess it should go to the homebrew-versions repo, as v8.3 is now a maintained branch of Coq existing in parallel with coq latest version (8.4).