Document Homebrew formula upgrade to 5.0.0

- 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
Merge pull request #677 from sbesson/homebrew_5.0.0

Document Homebrew formula upgrade to 5.0.0