Crash on 10.9

[dempseyatgithub]

Looks like the symbol QOS_CLASS_USER_INITIATED was introduced in 10.10, but the project has a deployment target of 10.9.

Running on 10.9 therefore crashes.

(I thought I should get a build warning for things like this. Filed rdar://20137077.)

Either bump the build target to 10.10 or conditionally use one symbol or the other.

[dempseyatgithub]

Or just use the 10.9 symbol, which is what I ended up doing to keep things simple.