Homebrew hudson script: CURL fix (rebased onto develop)



This is the same as gh-915 but rebased onto develop.

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This PR fixes a but reported by @nikola. When no `bin/pip` exists under `$BREW_DIR` the `CURL` variable used for downloading virtualenv was missing.

In the OMERO-homebrew-install.sh script, `$BREW_DIR/bin/pip` is now systematically deleted in order to test this cURL operation.