Loosen the pinning on "ohai"

Ohai requirement of ~> 8.0 is incompatible with Chef 13 or newer.

Unpin the requirement a little to allow for Ohai up to 14.x, as for
omnibus's needs the Ohai API should be stable up through Chef/Omnibus 14

Signed-off-by: Richard Clamp <richardc@unixbeard.net>

omnibus.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency "cleanroom",        "~> 1.0"
   gem.add_dependency "ffi-yajl",         "~> 2.2"
   gem.add_dependency "mixlib-shellout",  "~> 2.0"
-  gem.add_dependency "ohai",             "~> 8.0"
+  gem.add_dependency "ohai",             ">= 8.6.0.alpha.1", "< 15"
   gem.add_dependency "ruby-progressbar", "~> 1.7"
   gem.add_dependency "thor",             "~> 0.18"
   gem.add_dependency "license_scout",    "~> 1.0"