Refactor ICE_VERSION logic in the OMERO homebrew installation script

- Rely upon (brew deps omero $BREW_OPTS) to detect installed Ice version
- Remove OMERO_ALT variable (replaced by the scc merge logic in the library)
Merge pull request #1215 from sbesson/rebased/develop/homebrew_ice34

Refactor ICE_VERSION logic in the OMERO homebrew installation script (rebased onto develop)