Document Homebrew formula upgrade to 5.0.0 (rebased onto develop)



This is the same as gh-677 but rebased onto develop.

----

- Remove the `--devel` option to install OMERO 5.0.0
- Simplify the environment variable configuration for Ice 3.5 (default)
- Add environment variable sentence for non-default Ice 3.3 and 3.4