Add gh-pages checkout in make publish

[ak-2485]

Running make publish may result in an error like the following:

COQDOC -d html 
cp index.html html/index.html
cd gh-pages; git submodule update
fatal: destination path '/Users/arielkellison/Projects/LAProof/gh-pages' already exists and is not an empty directory.
fatal: clone of 'git@github.com:VeriNum/LAProof.git' into submodule path '/Users/arielkellison/Projects/LAProof/gh-pages' failed
Failed to clone 'gh-pages'. Retry scheduled
fatal: destination path '/Users/arielkellison/Projects/LAProof/gh-pages' already exists and is not an empty directory.
fatal: clone of 'git@github.com:VeriNum/LAProof.git' into submodule path '/Users/arielkellison/Projects/LAProof/gh-pages' failed
Failed to clone 'gh-pages' a second time, aborting
make[1]: *** [publish] Error 1
make: *** [invoke-coqmakefile] Error 2

Which can be fixed by removing the directory and reinitializing: rm -rf gh-pages && git submodule update --init .

But this leaves the submodule in a detached  HEAD state, so this PR updates Makefile.coq.local to add git checkout gh-pages.