Add genshi as a resource of the OME5 formulas

With recent changes to the way Homebrew handles Python, formulas were not building anymore. This PR  nows ships Genshi as a resource of the `--devel` versions of OMERO and Bio-Formats as recommended in  https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook#wiki-specifying-gems-python-modules-etc-as-dependencies