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

With this commit, `brew install omero` now installs Ice 3.4 by default. To draw back to an Ice 3.3 installation, use the `--with-ice33` option.

To test this PR;
- check the Travis status is green
- check the OMERO-homebrew-stable and OMERO-homebrew-develop jobs are passing with this PR merged in

Once this PR is merged in, we should update the Homebrew documentation page to reflect the change in the default installation.

/cc @rleigh-dundee, @bpindelski, @jburel, @hflynn