Fail with an error on Ruby 1.8 when posix-spawn is not present.

spec/foreman/cli_spec.rb

@@ -100,4 +100,12 @@ describe "Foreman::CLI", :fakefs do
     end
   end
 
+  describe "when posix-spawn is not present on ruby 1.8" do
+    it "should fail with an error" do
+      mock(Kernel).require('posix/spawn') { raise LoadError }
+      output = foreman("start -f #{resource_path("Procfile")}")
+      output.should == "ERROR: foreman requires gem `posix-spawn` on Ruby #{RUBY_VERSION}. Please `gem install posix-spawn`.\n"
+    end
+  end if running_ruby_18?
+
 end