Fix OMERO version number generation

- Fix a bug in the server upgrade page where the previous version number was
calculated to 5.-1
- Correctly parse the version numbers using semver.org conventions
- Define list of previous minor versions for each major version
- Do not import the re module globally
Restore use of the environment variable as the release number
Add explanations for the hard-coded get_previous_version function

Also wrap the error in an Exception() when previous version is undefined
Use tuples rather than lists
Add XQuartz instructions for 10.8
Fix Python dependencies installation script name
Merge pull request #367 from sbesson/rebased/dev_4_4/version

Fix OMERO version number generation (rebased onto dev_4_4)
Merge pull request #371 from sbesson/homebrew_xquartz

Homebrew xquartz