install node in cross-platform way + fix mapnik-packaging checkout
update mapnik-packaging to work around openssl problem

 - refs https://github.com/mapnik/mapnik-packaging/commit/6d7e83e8042a306d847a1e0b82b9bbe8eb76ccdd
update README with more iOS info
Merge remote-tracking branch 'origin/master' into dual-os-travis