Bump ffi version to 1.5.0
multiple dep lines no longer supported by bundler

our attempt to let the unix gemspec float with semver ~> 1.2 and to pin
the version exactly for windows (= 1.2.0) is no longer supported and
rake install blows up:

  if rspec-mocks is semantically versioned, use:
    add_development_dependency 'rspec-mocks', '~> 2.13', '>= 2.13.0'
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
rake aborted!
duplicate dependency on mixlib-shellout (= 1.2.0, ~> 1.2), (~> 1.2) use:
    add_runtime_dependency 'mixlib-shellout', '= 1.2.0, ~> 1.2', '~> 1.2'

Tasks: TOP => install => package => gem => pkg/chef-11.10.0.alpha.1-x86-mingw32.gem
(See full trace by running task with --trace)

just remove the line from the windows spec and inherit from the unix spec.
Resolve merge conflict with mixlib-shellout change
Merge pull request #1199 from opscode/adamed/chef-4913

CHEF-4913: ffi 1.3.1 is too low a version when using Ruby 2.0.0 with Windows