ant: Set SOURCE_BRANCH and OMERO_RELEASE using omero.shortversion
travis: Hardcode version for ant testing
Allow zero-length environment variables
Merge pull request #751 from rleigh-dundee/ant-props-dev50

ant: Set SOURCE_BRANCH and OMERO_RELEASE using omero.shortversion
Merge remote-tracking branch 'origin/dev_5_0' into ant-apisupport-dev50

Conflicts:
	common/rules.xml