Update Homebrew installation page to expose Ice 3.4 as the default version

- Describe Ice 3.4 as the default version (e.e. as installed using ``brew in
- Explicit alternate ways to build OMERO with Ice 3.3
- Fix environment variable to reflect the default choice
- Remove deprecated comment in the Ice 3.3 troubleshooting section
Fix envvar markup
Merge pull request #451 from sbesson/homebrew_default_ice

Update Homebrew installation page to expose Ice 3.4 as the default version