Use version 4.0.30 of passenger, which fixes Date header generation

cf. https://github.com/phusion/passenger/compare/release-4.0.29...release-4.0.30
cf. https://github.com/phusion/passenger/pull/93
Check for existence of symlink, not regular file

Previous behavior was to clobber an existing 'current' symlink, due to that symlink failing the `test -f` test since it wasn't a regular file.