Update dependency on GSL to 1.15.3

The previous version (0.6.3) fails to build on recent OS X.
However, the latest ocaml-gsl has moved to a packed module: `Gsl_vector` is
replaced by `Gsl.Vector`, for example.

This PR is the result of running:

    sed -i -e 's/Gsl_\([a-z]\)/Gsl.\U\1\E/g' $(find . -name \*.ml)

and fixing a few errors.

Up to you whether to merge: all tests pass and binaries compile, but it's a big diff.