enable to run on ruby 1.8.7 (el6)

* Force ruby 1.8.7 hashrocket style via...
  `bundle exec rubocop --only='Style/HashSyntax' -a`

* Handle differences in net/http codebase between 1.8 and 1.9+

* On ruby 1.8, use posix-spawn in test harness

* Exclude rubocop and coco in bundle on ruby 1.8.7 to
  avoid pulling in deps like celluloid and rainbow.
  (Treat ruby 1.8.7 as a runtime env; not devenv.)