Add one-liner workaround to prevent the Homebrew job failure

Due to incompatibilites between Pytables 3.1.1 and Cython > 0.2.0, the Python
dependencies installation systematically fails. Until a new version is
released, we need to force the installation of an older version of Cython.
Allow overriding of HDF5_DIR in the Python dependencies script
merge dev_5_0 -Dorg -Ssuccess-only: PR 3060 (upgrade the character encoding of passwords)
merge dev_5_0 -Dorg -Ssuccess-only: PR 3067 (Homebrew Cython 0.20)
merge dev_5_0 -Dorg -Ssuccess-only

Repository: openmicroscopy/openmicroscopy
Already up-to-date.

Merged PRs:
  # PR 3060 mtbc 'upgrade the character encoding of passwords'
  # PR 3067 sbesson 'Homebrew Cython 0.20'

Repository: openmicroscopy/bioformats
Updating f58463b..440626c
Previously merged:
  # PR 1355 snoopycrimecop 'Update auto-generated documentation'
  # PR 1354 melissalinkert 'Update POM version numbers to 5.0.6-SNAPSHOT'

Repository: ome/scripts
Already up-to-date.

Repository: openmicroscopy/ome-documentation
Updating b1db7b2..c092e5b
Previously merged:
  # PR 1001 hflynn '5.0.5 password note for upgrade page'
  # PR 997 sbesson 'Add extlink for security vulnerabilities pages'

Merged PRs:
  # PR 998 manics 'Update Linux walkthrough to 5.0.5'

Generated by OME-5.0-merge-push#224 (http://ci.openmicroscopy.org/job/OME-5.0-merge-push/224/)