Adjust for changes in PriorityQueue type in 0.4 (fix #7)

This seems to work on 0.3, and hopefully it will work on 0.4 too.

From `include("test_worker_set.jl")`, I do get
```
Mode FF PASSED
ERROR: test error during contains(defprocs,i)
`contains` has no method matching contains(::Array{Int64,1}, ::Int64)
 in anonymous at test.jl:62
 in do_test at test.jl:37
 in anonymous at no file:53
 in include at ./boot.jl:245
 in include_from_node1 at ./loading.jl:128
while loading /home/tim/.julia/v0.3/PTools/test/test_worker_set.jl, in expression starting on line 52
```
and lots of deprecation warnings. But that's true on `master` as well.