ruby-native: Workaround parallel make race

There is a race condition in the ruby-native package that does not
appear to be resolved upstream.

http://marc.info/?t=123428742700005&r=1&w=2

Work-around it by disabling parallel make for ruby.

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Merge pull request #25 from drewmoseley/fix-parallel-make-race

ruby-native: Workaround parallel make race