Set Ice 3.4 as the default Ice version in the OMERO formula

With this commit, `brew install omero` installs Ice 3.4 by default. To draw
back to an Ice 3.3 installation, use the `--with-ice33` option.
Merge pull request #42 from sbesson/omero_default_ice

Set Ice 3.4 as the default Ice version in the OMERO formula